diff --git a/Libraries/LibWeb/DOM/Document.cpp b/Libraries/LibWeb/DOM/Document.cpp
index 8dd6edf412..1c2073a205 100644
--- a/Libraries/LibWeb/DOM/Document.cpp
+++ b/Libraries/LibWeb/DOM/Document.cpp
@@ -674,6 +674,8 @@ void Document::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_hovered_node);
     visitor.visit(m_inspected_node);
     visitor.visit(m_highlighted_node);
+    for (auto const& flexbox_highlight : m_flexbox_highlights)
+        visitor.visit(flexbox_highlight.node);
     for (auto const& grid_highlight : m_grid_highlights)
         visitor.visit(grid_highlight.node);
     visitor.visit(m_active_favicon);
@@ -2671,6 +2673,44 @@ void Document::set_grid_highlighted_node(GC::Ptr<Node> node, Painting::GridInspe
     node->set_needs_repaint();
 }
 
+void Document::set_flexbox_highlighted_node(GC::Ptr<Node> node, Painting::FlexboxInspectorOverlayOptions options)
+{
+    if (!node)
+        return;
+
+    for (auto& flexbox_highlight : m_flexbox_highlights) {
+        if (flexbox_highlight.node != node)
+            continue;
+
+        flexbox_highlight.options = options;
+        node->set_needs_repaint();
+        return;
+    }
+
+    m_flexbox_highlights.append({ node, options });
+    node->set_needs_repaint();
+}
+
+void Document::clear_flexbox_highlighted_node(GC::Ptr<Node> node)
+{
+    if (!node) {
+        for (auto const& flexbox_highlight : m_flexbox_highlights) {
+            if (flexbox_highlight.node)
+                flexbox_highlight.node->set_needs_repaint();
+        }
+        m_flexbox_highlights.clear();
+        return;
+    }
+
+    auto old_size = m_flexbox_highlights.size();
+    m_flexbox_highlights.remove_all_matching([&](auto const& flexbox_highlight) {
+        return flexbox_highlight.node == node;
+    });
+
+    if (m_flexbox_highlights.size() != old_size)
+        node->set_needs_repaint();
+}
+
 void Document::clear_grid_highlighted_node(GC::Ptr<Node> node)
 {
     if (!node) {
@@ -8375,6 +8415,16 @@ RefPtr<Painting::DisplayList> Document::record_display_list(HTML::PaintConfig co
         highlighted_node()->paintable()->paint_inspector_overlay(context);
     }
 
+    for (auto const& flexbox_highlight : m_flexbox_highlights) {
+        if (!flexbox_highlight.node)
+            continue;
+        auto paintable = flexbox_highlight.node->paintable();
+        auto const* paintable_box = as_if<Painting::PaintableBox>(paintable.ptr());
+        if (!paintable_box)
+            continue;
+        paintable_box->paint_flexbox_inspector_overlay(context, flexbox_highlight.options);
+    }
+
     for (auto const& grid_highlight : m_grid_highlights) {
         if (!grid_highlight.node)
             continue;
diff --git a/Libraries/LibWeb/DOM/Document.h b/Libraries/LibWeb/DOM/Document.h
index 2e003f4c71..13aeca8987 100644
--- a/Libraries/LibWeb/DOM/Document.h
+++ b/Libraries/LibWeb/DOM/Document.h
@@ -47,6 +47,7 @@
 #include <LibWeb/HTML/SessionHistoryEntry.h>
 #include <LibWeb/HTML/VisibilityState.h>
 #include <LibWeb/InvalidateDisplayList.h>
+#include <LibWeb/Painting/FlexboxInspectorOverlay.h>
 #include <LibWeb/Painting/GridInspectorOverlay.h>
 #include <LibWeb/ResizeObserver/ResizeObserver.h>
 #include <LibWeb/TrustedTypes/InjectionSink.h>
@@ -308,6 +309,8 @@ public:
     GC::Ptr<Node const> highlighted_node() const { return m_highlighted_node; }
     GC::Ptr<Layout::Node> highlighted_layout_node();
     GC::Ptr<Layout::Node const> highlighted_layout_node() const { return const_cast<Document*>(this)->highlighted_layout_node(); }
+    void set_flexbox_highlighted_node(GC::Ptr<Node>, Painting::FlexboxInspectorOverlayOptions);
+    void clear_flexbox_highlighted_node(GC::Ptr<Node>);
     void set_grid_highlighted_node(GC::Ptr<Node>, Painting::GridInspectorOverlayOptions);
     void clear_grid_highlighted_node(GC::Ptr<Node>);
 
@@ -1214,6 +1217,13 @@ private:
         GC::Ptr<Node> node;
         Painting::GridInspectorOverlayOptions options;
     };
+
+    struct FlexboxHighlight {
+        GC::Ptr<Node> node;
+        Painting::FlexboxInspectorOverlayOptions options;
+    };
+
+    Vector<FlexboxHighlight> m_flexbox_highlights;
     Vector<GridHighlight> m_grid_highlights;
 
     Optional<Color> m_normal_link_color;
diff --git a/Libraries/LibWeb/Painting/FlexboxInspectorOverlay.h b/Libraries/LibWeb/Painting/FlexboxInspectorOverlay.h
new file mode 100644
index 0000000000..7f5ea555b2
--- /dev/null
+++ b/Libraries/LibWeb/Painting/FlexboxInspectorOverlay.h
@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2026, Ladybird contributors
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGfx/Color.h>
+
+namespace Web::Painting {
+
+struct FlexboxInspectorOverlayOptions {
+    Gfx::Color color { 148, 0, 255 };
+};
+
+}
diff --git a/Libraries/LibWeb/Painting/PaintableBox.cpp b/Libraries/LibWeb/Painting/PaintableBox.cpp
index 28cf2ffb6a..abc81cb809 100644
--- a/Libraries/LibWeb/Painting/PaintableBox.cpp
+++ b/Libraries/LibWeb/Painting/PaintableBox.cpp
@@ -31,6 +31,7 @@
 #include <LibWeb/Painting/ChromeMetrics.h>
 #include <LibWeb/Painting/DisplayListRecorder.h>
 #include <LibWeb/Painting/DisplayListRecordingContext.h>
+#include <LibWeb/Painting/FlexboxInspectorOverlay.h>
 #include <LibWeb/Painting/GridInspectorOverlay.h>
 #include <LibWeb/Painting/PaintableBox.h>
 #include <LibWeb/Painting/ResizeHandle.h>
@@ -1320,6 +1321,75 @@ void PaintableBox::paint_grid_inspector_overlay(DisplayListRecordingContext& con
     });
 }
 
+void PaintableBox::paint_flexbox_inspector_overlay(DisplayListRecordingContext& context, FlexboxInspectorOverlayOptions const& options) const
+{
+    if (!m_flex_layout_data)
+        return;
+
+    paint_with_inspector_overlay_context(context, [&] {
+        auto content_rect = absolute_united_content_rect();
+        auto const origin = content_rect.location();
+        auto const viewport_rect = document().viewport_rect();
+        auto const& color = options.color;
+        auto line_color = color.with_alpha(220);
+        auto container_fill_color = color.with_alpha(28);
+        auto line_fill_color = color.with_alpha(18);
+        auto item_fill_color = color.with_alpha(32);
+        auto line_thickness = CSSPixels(1);
+        auto main_axis_is_horizontal = m_flex_layout_data->flex_direction == CSS::FlexDirection::Row
+            || m_flex_layout_data->flex_direction == CSS::FlexDirection::RowReverse;
+
+        auto paint_rect = [&](CSSPixelRect const& rect, Gfx::Color rect_color) {
+            auto visible_rect = rect.intersected(viewport_rect);
+            if (visible_rect.is_empty())
+                return;
+            context.display_list_recorder().fill_rect(context.enclosing_device_rect(visible_rect).to_type<int>(), rect_color);
+        };
+
+        auto paint_outline = [&](CSSPixelRect const& rect, Gfx::Color rect_color) {
+            auto visible_rect = rect.intersected(viewport_rect);
+            if (visible_rect.is_empty())
+                return;
+            context.display_list_recorder().draw_rect(context.enclosing_device_rect(visible_rect).to_type<int>(), rect_color);
+        };
+
+        paint_rect(content_rect, container_fill_color);
+        paint_outline(content_rect, line_color);
+
+        for (auto const& line : m_flex_layout_data->lines) {
+            auto line_rect = main_axis_is_horizontal
+                ? CSSPixelRect { content_rect.x(), origin.y() + line.cross_start, content_rect.width(), line.cross_size }
+                : CSSPixelRect { origin.x() + line.cross_start, content_rect.y(), line.cross_size, content_rect.height() };
+
+            paint_rect(line_rect, line_fill_color);
+            paint_outline(line_rect, line_color);
+
+            for (auto const& item : line.items) {
+                auto item_rect = item.rect.translated(origin);
+                paint_rect(item_rect, item_fill_color);
+                paint_outline(item_rect, line_color);
+            }
+        }
+
+        // Repaint the container border last so adjacent flex lines and items do not obscure it.
+        paint_outline(content_rect, line_color);
+
+        if (main_axis_is_horizontal) {
+            for (auto const& line : m_flex_layout_data->lines) {
+                auto y = origin.y() + line.cross_start;
+                paint_rect({ content_rect.x(), y, content_rect.width(), line_thickness }, line_color);
+                paint_rect({ content_rect.x(), y + line.cross_size, content_rect.width(), line_thickness }, line_color);
+            }
+        } else {
+            for (auto const& line : m_flex_layout_data->lines) {
+                auto x = origin.x() + line.cross_start;
+                paint_rect({ x, content_rect.y(), line_thickness, content_rect.height() }, line_color);
+                paint_rect({ x + line.cross_size, content_rect.y(), line_thickness, content_rect.height() }, line_color);
+            }
+        }
+    });
+}
+
 void PaintableBox::set_stacking_context(NonnullRefPtr<StackingContext> stacking_context)
 {
     m_stacking_context = move(stacking_context);
diff --git a/Libraries/LibWeb/Painting/PaintableBox.h b/Libraries/LibWeb/Painting/PaintableBox.h
index 5db5944fd9..4f5090ec82 100644
--- a/Libraries/LibWeb/Painting/PaintableBox.h
+++ b/Libraries/LibWeb/Painting/PaintableBox.h
@@ -30,6 +30,7 @@
 
 namespace Web::Painting {
 
+struct FlexboxInspectorOverlayOptions;
 struct GridInspectorOverlayOptions;
 class ResizeHandle;
 class Scrollbar;
@@ -283,6 +284,7 @@ public:
     Layout::GridLayoutData const* grid_layout_data() const { return m_grid_layout_data.ptr(); }
     void set_flex_layout_data(OwnPtr<Layout::FlexLayoutData> flex_layout_data) { m_flex_layout_data = move(flex_layout_data); }
     Layout::FlexLayoutData const* flex_layout_data() const { return m_flex_layout_data.ptr(); }
+    void paint_flexbox_inspector_overlay(DisplayListRecordingContext&, FlexboxInspectorOverlayOptions const&) const;
     void paint_grid_inspector_overlay(DisplayListRecordingContext&, GridInspectorOverlayOptions const&) const;
 
     void set_enclosing_scroll_frame_index(ScrollFrameIndex index) { m_enclosing_scroll_frame_index = index; }