diff --git a/AK/CMakeLists.txt b/AK/CMakeLists.txt
index ffde0fdf1f..3dcbd19b1f 100644
--- a/AK/CMakeLists.txt
+++ b/AK/CMakeLists.txt
@@ -31,6 +31,7 @@ set(SOURCES
     Time.cpp
     Utf16FlyString.cpp
     Utf16String.cpp
+    Utf16StringBuilder.cpp
     Utf16StringData.cpp
     Utf16View.cpp
     Utf8View.cpp
diff --git a/AK/Forward.h b/AK/Forward.h
index d88397d236..400f872390 100644
--- a/AK/Forward.h
+++ b/AK/Forward.h
@@ -57,6 +57,7 @@ class StringView;
 class UnixDateTime;
 class Utf16FlyString;
 class Utf16String;
+class Utf16StringBuilder;
 class Utf16View;
 class Utf8CodePointIterator;
 class Utf8View;
diff --git a/AK/Utf16String.cpp b/AK/Utf16String.cpp
index d1b24f1da2..b984d8fb93 100644
--- a/AK/Utf16String.cpp
+++ b/AK/Utf16String.cpp
@@ -7,6 +7,7 @@
 #include <AK/Stream.h>
 #include <AK/StringNumber.h>
 #include <AK/Utf16String.h>
+#include <AK/Utf16StringBuilder.h>
 
 #include <simdutf.h>
 
@@ -103,6 +104,23 @@ Utf16String Utf16String::from_string_builder(Badge<StringBuilder>, StringBuilder
     return Utf16String { Detail::Utf16StringData::from_string_builder(builder) };
 }
 
+Utf16String Utf16String::from_string_builder(Badge<Utf16StringBuilder>, Utf16StringBuilder& builder)
+{
+    auto view = builder.view();
+
+    if (view.length_in_code_units() <= Detail::MAX_SHORT_STRING_BYTE_COUNT && view.has_ascii_storage()) {
+        Utf16String string;
+        string.m_value.short_ascii_string = Detail::ShortString::create_with_byte_count(view.length_in_code_units());
+
+        auto result = view.bytes().copy_to(string.m_value.short_ascii_string.storage);
+        VERIFY(result == view.length_in_code_units());
+
+        return string;
+    }
+
+    return Utf16String { Detail::Utf16StringData::from_string_builder(builder) };
+}
+
 ErrorOr<Utf16String> Utf16String::from_ipc_stream(Stream& stream, size_t length_in_code_units, bool is_ascii)
 {
     if (is_ascii && length_in_code_units <= Detail::MAX_SHORT_STRING_BYTE_COUNT) {
diff --git a/AK/Utf16String.h b/AK/Utf16String.h
index dd93b7daa5..1bf1380969 100644
--- a/AK/Utf16String.h
+++ b/AK/Utf16String.h
@@ -230,6 +230,7 @@ public:
     ALWAYS_INLINE Utf16String escape_html_entities() const { return utf16_view().escape_html_entities(); }
 
     static Utf16String from_string_builder(Badge<StringBuilder>, StringBuilder& builder);
+    static Utf16String from_string_builder(Badge<Utf16StringBuilder>, Utf16StringBuilder& builder);
     static ErrorOr<Utf16String> from_ipc_stream(Stream&, size_t length_in_code_units, bool is_ascii);
 
     constexpr Utf16String(Badge<Optional<Utf16String>>, nullptr_t)
diff --git a/AK/Utf16StringBuilder.cpp b/AK/Utf16StringBuilder.cpp
new file mode 100644
index 0000000000..11cd7de1bf
--- /dev/null
+++ b/AK/Utf16StringBuilder.cpp
@@ -0,0 +1,409 @@
+/*
+ * Copyright (c) 2026-present, the Ladybird developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/CharacterTypes.h>
+#include <AK/Checked.h>
+#include <AK/UnicodeUtils.h>
+#include <AK/Utf16String.h>
+#include <AK/Utf16StringBuilder.h>
+#include <AK/Utf16StringData.h>
+#include <AK/kmalloc.h>
+
+namespace AK {
+
+Utf16StringBuilder::Buffer::~Buffer()
+{
+    clear();
+}
+
+u8* Utf16StringBuilder::Buffer::data()
+{
+    return m_inline ? m_inline_buffer : m_outline_buffer;
+}
+
+u8 const* Utf16StringBuilder::Buffer::data() const
+{
+    return m_inline ? m_inline_buffer : m_outline_buffer;
+}
+
+Bytes Utf16StringBuilder::Buffer::span()
+{
+    return { data(), size() };
+}
+
+ReadonlyBytes Utf16StringBuilder::Buffer::span() const
+{
+    return { data(), size() };
+}
+
+void Utf16StringBuilder::Buffer::clear()
+{
+    if (!m_inline) {
+        kfree(m_outline_buffer);
+        m_inline = true;
+    }
+    m_size = 0;
+}
+
+void Utf16StringBuilder::Buffer::resize(size_t new_size)
+{
+    if (new_size <= m_size) {
+        trim(new_size, false);
+        return;
+    }
+
+    ensure_capacity(new_size);
+    set_size(new_size);
+}
+
+void Utf16StringBuilder::Buffer::set_size(size_t new_size)
+{
+    ASSERT(new_size <= capacity());
+    m_size = new_size;
+}
+
+void Utf16StringBuilder::Buffer::ensure_capacity(size_t new_capacity)
+{
+    if (new_capacity <= capacity())
+        return;
+    ensure_capacity_slowpath(new_capacity);
+}
+
+void Utf16StringBuilder::Buffer::append(void const* data, size_t data_size)
+{
+    if (data_size == 0)
+        return;
+    VERIFY(data != nullptr);
+
+    auto old_size = size();
+    Checked<size_t> new_size = old_size;
+    new_size += data_size;
+    VERIFY(!new_size.has_overflow());
+
+    resize(new_size.value());
+    __builtin_memcpy(this->data() + old_size, data, data_size);
+}
+
+auto Utf16StringBuilder::Buffer::leak_outline_buffer() -> Optional<OutlineBuffer>
+{
+    if (m_inline)
+        return {};
+
+    auto* outline_buffer = m_outline_buffer;
+    auto size = m_size;
+    auto outline_capacity = m_outline_capacity;
+
+    m_inline = true;
+    m_size = 0;
+
+    return OutlineBuffer { Bytes { outline_buffer, size }, outline_capacity };
+}
+
+void Utf16StringBuilder::Buffer::trim(size_t size, bool may_discard_existing_data)
+{
+    VERIFY(size <= m_size);
+    if (!m_inline && size <= inline_capacity)
+        shrink_into_inline_buffer(size, may_discard_existing_data);
+    m_size = size;
+}
+
+void Utf16StringBuilder::Buffer::shrink_into_inline_buffer(size_t size, bool may_discard_existing_data)
+{
+    auto* outline_buffer = m_outline_buffer;
+    if (!may_discard_existing_data)
+        __builtin_memcpy(m_inline_buffer, outline_buffer, size);
+    kfree(outline_buffer);
+    m_inline = true;
+}
+
+void Utf16StringBuilder::Buffer::ensure_capacity_slowpath(size_t new_capacity)
+{
+    Checked<size_t> grown_capacity = capacity();
+    grown_capacity *= 3;
+    VERIFY(!grown_capacity.has_overflow());
+
+    new_capacity = max(new_capacity, grown_capacity.value() / 2);
+    new_capacity = kmalloc_good_size(new_capacity);
+
+    if (m_inline) {
+        auto* new_buffer = static_cast<u8*>(kmalloc(HeapPartition::String, new_capacity));
+        VERIFY(new_buffer);
+
+        __builtin_memcpy(new_buffer, data(), m_size);
+        m_outline_buffer = new_buffer;
+    } else {
+        auto* new_buffer = static_cast<u8*>(krealloc(HeapPartition::String, m_outline_buffer, new_capacity));
+        VERIFY(new_buffer);
+
+        m_outline_buffer = new_buffer;
+    }
+
+    m_outline_capacity = new_capacity;
+    m_inline = false;
+}
+
+static constexpr size_t string_builder_prefix_size()
+{
+    return Detail::Utf16StringData::offset_of_string_storage();
+}
+
+static size_t utf16_byte_count(size_t length_in_code_units)
+{
+    Checked<size_t> byte_count = length_in_code_units;
+    byte_count *= sizeof(char16_t);
+    VERIFY(!byte_count.has_overflow());
+    return byte_count.value();
+}
+
+Utf16StringBuilder::Utf16StringBuilder()
+{
+    initialize_buffer(inline_capacity / sizeof(char16_t));
+}
+
+Utf16StringBuilder::Utf16StringBuilder(size_t initial_capacity_in_code_units)
+{
+    initialize_buffer(initial_capacity_in_code_units);
+}
+
+void Utf16StringBuilder::initialize_buffer(size_t capacity_in_code_units)
+{
+    auto prefix_size = string_builder_prefix_size();
+    auto capacity_in_bytes = utf16_byte_count(capacity_in_code_units);
+
+    if (capacity_in_bytes > inline_capacity) {
+        Checked<size_t> capacity = prefix_size;
+        capacity += capacity_in_bytes;
+        VERIFY(!capacity.has_overflow());
+
+        m_buffer.ensure_capacity(capacity.value());
+    }
+    m_buffer.resize(prefix_size);
+}
+
+void Utf16StringBuilder::will_append(size_t size_in_bytes)
+{
+    Checked<size_t> needed_capacity = m_buffer.size();
+    needed_capacity += size_in_bytes;
+    VERIFY(!needed_capacity.has_overflow());
+
+    if (needed_capacity <= m_buffer.capacity())
+        return;
+
+    Checked<size_t> expanded_capacity = needed_capacity;
+    expanded_capacity *= 2;
+    VERIFY(!expanded_capacity.has_overflow());
+
+    m_buffer.ensure_capacity(expanded_capacity.value());
+}
+
+void Utf16StringBuilder::ensure_storage_is_utf16()
+{
+    if (!m_is_ascii)
+        return;
+
+    auto ascii_length = length_in_bytes();
+    m_is_ascii = false;
+    if (ascii_length == 0)
+        return;
+
+    m_buffer.resize(m_buffer.size() + ascii_length);
+
+    Bytes source { data(), ascii_length };
+    Span<char16_t> target { reinterpret_cast<char16_t*>(data()), ascii_length };
+
+    for (size_t i = ascii_length; i > 0; --i) {
+        auto index = i - 1;
+
+        auto code_unit = static_cast<char16_t>(source[index]);
+        target[index] = code_unit;
+    }
+}
+
+void Utf16StringBuilder::append(Utf16View const& view)
+{
+    if (view.is_empty())
+        return;
+
+    if (m_is_ascii && view.is_ascii()) {
+        will_append(view.length_in_code_units());
+
+        if (view.has_ascii_storage()) {
+            m_buffer.append(view.ascii_span().data(), view.length_in_code_units());
+        } else {
+            for (auto code_unit : view.utf16_span()) {
+                auto ascii_code_unit = static_cast<char>(code_unit);
+                m_buffer.append(&ascii_code_unit, sizeof(ascii_code_unit));
+            }
+        }
+
+        return;
+    }
+
+    ensure_storage_is_utf16();
+    will_append(utf16_byte_count(view.length_in_code_units()));
+
+    if (view.has_ascii_storage()) {
+        for (auto code_unit : view.ascii_span()) {
+            auto utf16_code_unit = static_cast<char16_t>(code_unit);
+            m_buffer.append(&utf16_code_unit, sizeof(utf16_code_unit));
+        }
+    } else {
+        m_buffer.append(view.utf16_span().data(), utf16_byte_count(view.length_in_code_units()));
+    }
+}
+
+void Utf16StringBuilder::append_ascii(char code_unit)
+{
+    VERIFY(is_ascii(code_unit));
+
+    if (m_is_ascii) {
+        will_append(1);
+        m_buffer.append(&code_unit, sizeof(code_unit));
+    } else {
+        auto utf16_code_unit = static_cast<char16_t>(code_unit);
+        will_append(sizeof(utf16_code_unit));
+        m_buffer.append(&utf16_code_unit, sizeof(utf16_code_unit));
+    }
+}
+
+void Utf16StringBuilder::append_ascii(StringView string)
+{
+    VERIFY(string.is_ascii());
+    if (string.is_empty())
+        return;
+
+    if (m_is_ascii) {
+        will_append(string.length());
+        m_buffer.append(string.characters_without_null_termination(), string.length());
+        return;
+    }
+
+    will_append(utf16_byte_count(string.length()));
+    for (auto code_unit : string) {
+        auto utf16_code_unit = static_cast<char16_t>(code_unit);
+        m_buffer.append(&utf16_code_unit, sizeof(utf16_code_unit));
+    }
+}
+
+void Utf16StringBuilder::append_code_unit(char16_t code_unit)
+{
+    if (m_is_ascii && is_ascii(code_unit)) {
+        append_ascii(static_cast<char>(code_unit));
+        return;
+    }
+
+    ensure_storage_is_utf16();
+    will_append(sizeof(code_unit));
+    m_buffer.append(&code_unit, sizeof(code_unit));
+}
+
+void Utf16StringBuilder::append_code_point(u32 code_point)
+{
+    if (!is_unicode(code_point)) {
+        append_code_point(UnicodeUtils::REPLACEMENT_CODE_POINT);
+        return;
+    }
+
+    if (m_is_ascii && is_ascii(code_point)) {
+        append_ascii(static_cast<char>(code_point));
+        return;
+    }
+
+    ensure_storage_is_utf16();
+
+    (void)UnicodeUtils::code_point_to_utf16(code_point, [this](char16_t code_unit) {
+        will_append(sizeof(code_unit));
+        m_buffer.append(&code_unit, sizeof(code_unit));
+    });
+}
+
+void Utf16StringBuilder::append_repeated_ascii(char code_unit, size_t count)
+{
+    VERIFY(is_ascii(code_unit));
+
+    if (m_is_ascii) {
+        will_append(count);
+        for (size_t i = 0; i < count; ++i)
+            m_buffer.append(&code_unit, sizeof(code_unit));
+        return;
+    }
+
+    auto utf16_code_unit = static_cast<char16_t>(code_unit);
+    will_append(utf16_byte_count(count));
+    for (size_t i = 0; i < count; ++i)
+        m_buffer.append(&utf16_code_unit, sizeof(utf16_code_unit));
+}
+
+void Utf16StringBuilder::append_repeated(Utf16View const& view, size_t count)
+{
+    for (size_t i = 0; i < count; ++i)
+        append(view);
+}
+
+Utf16String Utf16StringBuilder::to_string()
+{
+    return Utf16String::from_string_builder({}, *this);
+}
+
+Utf16View Utf16StringBuilder::view() const
+{
+    auto data = m_buffer.span().slice(string_builder_prefix_size());
+
+    if (m_is_ascii)
+        return { reinterpret_cast<char const*>(data.data()), data.size() };
+    return { reinterpret_cast<char16_t const*>(data.data()), data.size() / sizeof(char16_t) };
+}
+
+size_t Utf16StringBuilder::length_in_code_units() const
+{
+    auto length = length_in_bytes();
+    if (m_is_ascii)
+        return length;
+    return length / sizeof(char16_t);
+}
+
+void Utf16StringBuilder::trim(size_t count_in_code_units)
+{
+    auto count_in_bytes = count_in_code_units;
+    if (!m_is_ascii)
+        count_in_bytes = utf16_byte_count(count_in_code_units);
+
+    auto decrease_count = min(length_in_bytes(), count_in_bytes);
+    m_buffer.resize(m_buffer.size() - decrease_count);
+}
+
+void Utf16StringBuilder::clear()
+{
+    m_buffer.resize(string_builder_prefix_size());
+    m_is_ascii = true;
+}
+
+auto Utf16StringBuilder::leak_buffer_for_string_construction() -> Optional<Buffer::OutlineBuffer>
+{
+    if (auto buffer = m_buffer.leak_outline_buffer(); buffer.has_value()) {
+        clear();
+        return buffer;
+    }
+
+    return {};
+}
+
+u8* Utf16StringBuilder::data()
+{
+    return m_buffer.data() + string_builder_prefix_size();
+}
+
+u8 const* Utf16StringBuilder::data() const
+{
+    return m_buffer.data() + string_builder_prefix_size();
+}
+
+size_t Utf16StringBuilder::length_in_bytes() const
+{
+    return m_buffer.size() - string_builder_prefix_size();
+}
+
+}
diff --git a/AK/Utf16StringBuilder.h b/AK/Utf16StringBuilder.h
new file mode 100644
index 0000000000..ae6b476d49
--- /dev/null
+++ b/AK/Utf16StringBuilder.h
@@ -0,0 +1,120 @@
+/*
+ * Copyright (c) 2026-present, the Ladybird developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Badge.h>
+#include <AK/Forward.h>
+#include <AK/Optional.h>
+#include <AK/Span.h>
+#include <AK/StringView.h>
+#include <AK/Types.h>
+#include <AK/Utf16View.h>
+
+namespace AK {
+
+class Utf16StringBuilder {
+public:
+    static constexpr size_t inline_capacity = 256;
+
+    class Buffer {
+    public:
+        Buffer() = default;
+        ~Buffer();
+
+        Buffer(Buffer const&) = delete;
+        Buffer(Buffer&&) = delete;
+        Buffer& operator=(Buffer const&) = delete;
+        Buffer& operator=(Buffer&&) = delete;
+
+        [[nodiscard]] u8* data();
+        [[nodiscard]] u8 const* data() const;
+
+        [[nodiscard]] Bytes span() LIFETIME_BOUND;
+        [[nodiscard]] ReadonlyBytes span() const LIFETIME_BOUND;
+
+        [[nodiscard]] size_t size() const { return m_size; }
+        [[nodiscard]] size_t capacity() const { return m_inline ? inline_capacity : m_outline_capacity; }
+        [[nodiscard]] bool is_inline() const { return m_inline; }
+
+        void clear();
+        void resize(size_t);
+        void set_size(size_t);
+        void ensure_capacity(size_t);
+        void append(void const*, size_t);
+
+        struct OutlineBuffer {
+            Bytes buffer;
+            size_t capacity { 0 };
+        };
+        Optional<OutlineBuffer> leak_outline_buffer();
+
+    private:
+        void trim(size_t, bool may_discard_existing_data);
+        void shrink_into_inline_buffer(size_t, bool may_discard_existing_data);
+        void ensure_capacity_slowpath(size_t);
+
+        union {
+            u8 m_inline_buffer[inline_capacity];
+            struct {
+                u8* m_outline_buffer;
+                size_t m_outline_capacity;
+            };
+        };
+        size_t m_size { 0 };
+        bool m_inline { true };
+    };
+
+    Utf16StringBuilder();
+    explicit Utf16StringBuilder(size_t initial_capacity_in_code_units);
+    ~Utf16StringBuilder() = default;
+
+    Utf16StringBuilder(Utf16StringBuilder const&) = delete;
+    Utf16StringBuilder(Utf16StringBuilder&&) = delete;
+    Utf16StringBuilder& operator=(Utf16StringBuilder const&) = delete;
+    Utf16StringBuilder& operator=(Utf16StringBuilder&&) = delete;
+
+    void append(Utf16View const&);
+    void append_ascii(char);
+    void append_ascii(StringView);
+    void append_code_unit(char16_t);
+    void append_code_point(u32);
+    void append_repeated_ascii(char, size_t);
+    void append_repeated(Utf16View const&, size_t);
+
+    [[nodiscard]] Utf16String to_string();
+
+    [[nodiscard]] Utf16View view() const;
+    [[nodiscard]] size_t length_in_code_units() const;
+    [[nodiscard]] bool is_empty() const { return length_in_code_units() == 0; }
+    void trim(size_t count_in_code_units);
+    void clear();
+
+    Optional<Buffer::OutlineBuffer> leak_buffer_for_string_construction(Badge<Detail::Utf16StringData>)
+    {
+        return leak_buffer_for_string_construction();
+    }
+
+private:
+    void initialize_buffer(size_t capacity_in_code_units);
+    void will_append(size_t size_in_bytes);
+    void ensure_storage_is_utf16();
+
+    Optional<Buffer::OutlineBuffer> leak_buffer_for_string_construction();
+
+    [[nodiscard]] u8* data();
+    [[nodiscard]] u8 const* data() const;
+    [[nodiscard]] size_t length_in_bytes() const;
+
+    Buffer m_buffer;
+    bool m_is_ascii { true };
+};
+
+}
+
+#if USING_AK_GLOBALLY
+using AK::Utf16StringBuilder;
+#endif
diff --git a/AK/Utf16StringData.cpp b/AK/Utf16StringData.cpp
index 9337dc4f8d..eea93d5e2c 100644
--- a/AK/Utf16StringData.cpp
+++ b/AK/Utf16StringData.cpp
@@ -7,6 +7,7 @@
 #include <AK/RefPtr.h>
 #include <AK/Stream.h>
 #include <AK/TypedTransfer.h>
+#include <AK/Utf16StringBuilder.h>
 #include <AK/Utf16StringData.h>
 #include <AK/Utf8View.h>
 
@@ -136,6 +137,33 @@ NonnullRefPtr<Utf16StringData> Utf16StringData::from_string_builder(StringBuilde
     return string.release_nonnull();
 }
 
+NonnullRefPtr<Utf16StringData> Utf16StringData::from_string_builder(Utf16StringBuilder& builder)
+{
+    auto view = builder.view();
+
+    auto code_unit_length = view.length_in_code_units();
+    VERIFY_UTF16_LENGTH(code_unit_length);
+
+    RefPtr<Utf16StringData> string;
+
+    if (auto buffer = builder.leak_buffer_for_string_construction(Badge<Utf16StringData> {}); buffer.has_value()) {
+        auto storage_type = view.has_ascii_storage() ? StorageType::ASCII : StorageType::UTF16;
+        string = adopt_ref(*new (buffer->buffer.data()) Utf16StringData { storage_type, code_unit_length });
+    } else {
+        if (view.has_ascii_storage()) {
+            string = create_uninitialized(StorageType::ASCII, code_unit_length);
+            TypedTransfer<char>::copy(string->m_ascii_data, view.ascii_span().data(), code_unit_length);
+        } else {
+            string = create_uninitialized(StorageType::UTF16, code_unit_length);
+            TypedTransfer<char16_t>::copy(string->m_utf16_data, view.utf16_span().data(), code_unit_length);
+
+            string->m_length_in_code_points = view.m_length_in_code_points;
+        }
+    }
+
+    return string.release_nonnull();
+}
+
 ErrorOr<NonnullRefPtr<Utf16StringData>> Utf16StringData::from_ipc_stream(Stream& stream, size_t length_in_code_units, bool is_ascii)
 {
     RefPtr<Utf16StringData> string;
diff --git a/AK/Utf16StringData.h b/AK/Utf16StringData.h
index f39fab1211..14cda660c1 100644
--- a/AK/Utf16StringData.h
+++ b/AK/Utf16StringData.h
@@ -35,6 +35,7 @@ public:
     static NonnullRefPtr<Utf16StringData> from_ascii(ReadonlyBytes);
     static NonnullRefPtr<Utf16StringData> from_utf16(Utf16View const&);
     static NonnullRefPtr<Utf16StringData> from_string_builder(StringBuilder&);
+    static NonnullRefPtr<Utf16StringData> from_string_builder(Utf16StringBuilder&);
     static ErrorOr<NonnullRefPtr<Utf16StringData>> from_ipc_stream(Stream&, size_t length_in_code_units, bool is_ascii);
 
     static NonnullRefPtr<Utf16StringData> to_well_formed(Utf16View const&);
diff --git a/AK/Utf16View.h b/AK/Utf16View.h
index 3660174a19..78b67bdc34 100644
--- a/AK/Utf16View.h
+++ b/AK/Utf16View.h
@@ -660,6 +660,7 @@ public:
 
 private:
     friend StringBuilder;
+    friend Utf16StringBuilder;
     friend Detail::Utf16StringBase;
     friend Detail::Utf16StringData;
 
diff --git a/Tests/AK/CMakeLists.txt b/Tests/AK/CMakeLists.txt
index 19395c9e77..d482af9d0c 100644
--- a/Tests/AK/CMakeLists.txt
+++ b/Tests/AK/CMakeLists.txt
@@ -74,6 +74,7 @@ set(AK_TEST_SOURCES
     TestUFixedBigInt.cpp
     TestUtf16FlyString.cpp
     TestUtf16String.cpp
+    TestUtf16StringBuilder.cpp
     TestUtf16View.cpp
     TestUtf8View.cpp
     TestVariant.cpp
diff --git a/Tests/AK/TestUtf16StringBuilder.cpp b/Tests/AK/TestUtf16StringBuilder.cpp
new file mode 100644
index 0000000000..e11e0dcdd1
--- /dev/null
+++ b/Tests/AK/TestUtf16StringBuilder.cpp
@@ -0,0 +1,110 @@
+/*
+ * Copyright (c) 2026-present, the Ladybird developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Utf16String.h>
+#include <AK/Utf16StringBuilder.h>
+#include <LibTest/TestCase.h>
+
+TEST_CASE(build_ascii_string)
+{
+    Utf16StringBuilder builder;
+    builder.append_ascii("hello"sv);
+    builder.append_ascii('!');
+
+    EXPECT_EQ(builder.view(), "hello!"sv);
+
+    auto string = builder.to_string();
+    EXPECT_EQ(string, "hello!"sv);
+    EXPECT(string.has_short_ascii_storage());
+}
+
+TEST_CASE(widen_to_utf16_storage)
+{
+    Utf16StringBuilder builder;
+    builder.append_ascii("hello "sv);
+    builder.append_code_unit(0x263A);
+
+    EXPECT_EQ(builder.view(), u"hello \u263A"sv);
+
+    auto string = builder.to_string();
+    EXPECT_EQ(string, u"hello \u263A"sv);
+    EXPECT(!string.is_ascii());
+}
+
+TEST_CASE(append_code_point)
+{
+    Utf16StringBuilder builder;
+    builder.append_ascii("wave "sv);
+    builder.append_code_point(0x1F44B);
+
+    auto string = builder.to_string();
+    EXPECT_EQ(string, u"wave \U0001F44B"sv);
+    EXPECT_EQ(string.length_in_code_units(), 7uz);
+    EXPECT_EQ(string.length_in_code_points(), 6uz);
+}
+
+TEST_CASE(trim_ascii_storage)
+{
+    Utf16StringBuilder builder;
+    builder.append_ascii("abc"sv);
+
+    builder.trim(1);
+
+    EXPECT_EQ(builder.view(), "ab"sv);
+    EXPECT_EQ(builder.to_string(), "ab"sv);
+}
+
+TEST_CASE(trim_utf16_storage)
+{
+    Utf16StringBuilder builder;
+    builder.append(u"ab\u0100"sv);
+
+    builder.trim(1);
+
+    EXPECT_EQ(builder.view(), "ab"sv);
+    EXPECT_EQ(builder.to_string(), "ab"sv);
+}
+
+TEST_CASE(long_ascii_string)
+{
+    Utf16StringBuilder builder;
+    builder.append_repeated_ascii('x', 300);
+
+    auto string = builder.to_string();
+    EXPECT_EQ(string.length_in_code_units(), 300uz);
+    EXPECT(string.has_long_ascii_storage());
+    EXPECT_EQ(string.utf16_view().code_unit_at(0), 'x');
+    EXPECT_EQ(string.utf16_view().code_unit_at(299), 'x');
+}
+
+TEST_CASE(long_utf16_string)
+{
+    Utf16StringBuilder builder;
+    builder.append_repeated(u"\u0100"sv, 300);
+
+    auto string = builder.to_string();
+    EXPECT_EQ(string.length_in_code_units(), 300uz);
+    EXPECT(!string.is_ascii());
+    EXPECT_EQ(string.utf16_view().code_unit_at(0), 0x0100);
+    EXPECT_EQ(string.utf16_view().code_unit_at(299), 0x0100);
+}
+
+TEST_CASE(reuse_after_creating_long_utf16_string)
+{
+    Utf16StringBuilder builder;
+    builder.append_repeated(u"\u0100"sv, 300);
+
+    auto string = builder.to_string();
+    EXPECT_EQ(string.length_in_code_units(), 300uz);
+    EXPECT(!string.is_ascii());
+
+    EXPECT(builder.is_empty());
+
+    builder.append_ascii("ok"sv);
+
+    EXPECT_EQ(builder.view(), "ok"sv);
+    EXPECT_EQ(builder.to_string(), "ok"sv);
+}