diff --git a/Libraries/LibWeb/CMakeLists.txt b/Libraries/LibWeb/CMakeLists.txt
index 0e62146fe18..5f9d1cac8b5 100644
--- a/Libraries/LibWeb/CMakeLists.txt
+++ b/Libraries/LibWeb/CMakeLists.txt
@@ -116,6 +116,7 @@ set(SOURCES
     CSS/CSSLayerBlockRule.cpp
     CSS/CSSLayerStatementRule.cpp
     CSS/CSSMarginRule.cpp
+    CSS/CSSMathNegate.cpp
     CSS/CSSMathProduct.cpp
     CSS/CSSMathSum.cpp
     CSS/CSSMathValue.cpp
diff --git a/Libraries/LibWeb/CSS/CSSMathNegate.cpp b/Libraries/LibWeb/CSS/CSSMathNegate.cpp
new file mode 100644
index 00000000000..fd214b510b6
--- /dev/null
+++ b/Libraries/LibWeb/CSS/CSSMathNegate.cpp
@@ -0,0 +1,92 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CSSMathNegate.h"
+#include <LibWeb/Bindings/CSSMathNegatePrototype.h>
+#include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/WebIDL/ExceptionOr.h>
+
+namespace Web::CSS {
+
+GC_DEFINE_ALLOCATOR(CSSMathNegate);
+
+GC::Ref<CSSMathNegate> CSSMathNegate::create(JS::Realm& realm, NumericType type, GC::Ref<CSSNumericValue> values)
+{
+    return realm.create<CSSMathNegate>(realm, move(type), move(values));
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#dom-cssmathnegate-cssmathnegate
+WebIDL::ExceptionOr<GC::Ref<CSSMathNegate>> CSSMathNegate::construct_impl(JS::Realm& realm, CSSNumberish value)
+{
+    // The CSSMathNegate(arg) constructor must, when called, perform the following steps:
+    // 1. Replace arg with the result of rectifying a numberish value for arg.
+    auto converted_value = rectify_a_numberish_value(realm, value);
+
+    // 2. Return a new CSSMathNegate whose value internal slot is set to arg.
+    return CSSMathNegate::create(realm, converted_value->type(), converted_value);
+}
+
+CSSMathNegate::CSSMathNegate(JS::Realm& realm, NumericType type, GC::Ref<CSSNumericValue> values)
+    : CSSMathValue(realm, Bindings::CSSMathOperator::Negate, move(type))
+    , m_value(move(values))
+{
+}
+
+CSSMathNegate::~CSSMathNegate() = default;
+
+void CSSMathNegate::initialize(JS::Realm& realm)
+{
+    WEB_SET_PROTOTYPE_FOR_INTERFACE(CSSMathNegate);
+    Base::initialize(realm);
+}
+
+void CSSMathNegate::visit_edges(Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_value);
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#serialize-a-cssmathvalue
+String CSSMathNegate::serialize_math_value(Nested nested, Parens parens) const
+{
+    // NB: Only steps 1 and 4 apply here.
+    // 1. Let s initially be the empty string.
+    StringBuilder s;
+
+    // 4. Otherwise, if this is a CSSMathNegate:
+    {
+        // 1. If paren-less is true, continue to the next step; otherwise, if nested is true, append "(" to s;
+        //    otherwise, append "calc(" to s.
+        if (parens == Parens::With) {
+            if (nested == Nested::Yes) {
+                s.append("("sv);
+            } else {
+                s.append("calc("sv);
+            }
+        }
+
+        // 2. Append "-" to s.
+        s.append("-"sv);
+
+        // 3. Serialize this’s value internal slot with nested set to true, and append the result to s.
+        s.append(m_value->to_string({ .nested = true }));
+
+        // 4. If paren-less is false, append ")" to s,
+        if (parens == Parens::With)
+            s.append(")"sv);
+
+        // 5. Return s.
+        return s.to_string_without_validation();
+    }
+}
+
+// https://drafts.css-houdini.org/css-typed-om-1/#dom-cssmathnegate-value
+GC::Ref<CSSNumericValue> CSSMathNegate::value() const
+{
+    return m_value;
+}
+
+}
diff --git a/Libraries/LibWeb/CSS/CSSMathNegate.h b/Libraries/LibWeb/CSS/CSSMathNegate.h
new file mode 100644
index 00000000000..4470f74a5df
--- /dev/null
+++ b/Libraries/LibWeb/CSS/CSSMathNegate.h
@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/CSSMathValue.h>
+
+namespace Web::CSS {
+
+// https://drafts.css-houdini.org/css-typed-om-1/#cssmathnegate
+class CSSMathNegate : public CSSMathValue {
+    WEB_PLATFORM_OBJECT(CSSMathNegate, CSSMathValue);
+    GC_DECLARE_ALLOCATOR(CSSMathNegate);
+
+public:
+    [[nodiscard]] static GC::Ref<CSSMathNegate> create(JS::Realm&, NumericType, GC::Ref<CSSNumericValue>);
+    static WebIDL::ExceptionOr<GC::Ref<CSSMathNegate>> construct_impl(JS::Realm&, CSSNumberish);
+
+    virtual ~CSSMathNegate() override;
+
+    virtual void initialize(JS::Realm&) override;
+    virtual void visit_edges(Visitor&) override;
+
+    GC::Ref<CSSNumericValue> value() const;
+
+    virtual String serialize_math_value(Nested, Parens) const override;
+
+private:
+    CSSMathNegate(JS::Realm&, NumericType, GC::Ref<CSSNumericValue>);
+    GC::Ref<CSSNumericValue> m_value;
+};
+
+}
diff --git a/Libraries/LibWeb/CSS/CSSMathNegate.idl b/Libraries/LibWeb/CSS/CSSMathNegate.idl
new file mode 100644
index 00000000000..278df117a38
--- /dev/null
+++ b/Libraries/LibWeb/CSS/CSSMathNegate.idl
@@ -0,0 +1,9 @@
+#import <CSS/CSSMathValue.idl>
+#import <CSS/CSSNumericValue.idl>
+
+// https://drafts.css-houdini.org/css-typed-om-1/#cssmathnegate
+[Exposed=(Window, Worker, PaintWorklet, LayoutWorklet)]
+interface CSSMathNegate : CSSMathValue {
+    constructor(CSSNumberish arg);
+    readonly attribute CSSNumericValue value;
+};
diff --git a/Libraries/LibWeb/CSS/CSSMathSum.cpp b/Libraries/LibWeb/CSS/CSSMathSum.cpp
index acfcfdf8d21..bd60e175ade 100644
--- a/Libraries/LibWeb/CSS/CSSMathSum.cpp
+++ b/Libraries/LibWeb/CSS/CSSMathSum.cpp
@@ -7,6 +7,7 @@
 #include "CSSMathSum.h"
 #include <LibWeb/Bindings/CSSMathSumPrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/CSS/CSSMathNegate.h>
 #include <LibWeb/CSS/CSSNumericArray.h>
 #include <LibWeb/WebIDL/DOMException.h>
 #include <LibWeb/WebIDL/ExceptionOr.h>
@@ -109,10 +110,9 @@ String CSSMathSum::serialize_math_value(Nested nested, Parens parens) const
 
             // 1. If arg is a CSSMathNegate, append " - " to s, then serialize arg’s value internal slot with nested
             //    set to true, and append the result to s.
-            // FIXME: Detect CSSMathNegate once that's implemented.
-            if (false) {
+            if (auto* negate = as_if<CSSMathNegate>(*arg)) {
                 s.append(" - "sv);
-                s.append(arg->to_string({ .nested = true }));
+                s.append(negate->value()->to_string({ .nested = true }));
             }
 
             // 2. Otherwise, append " + " to s, then serialize arg with nested set to true, and append the result to s.
diff --git a/Libraries/LibWeb/Forward.h b/Libraries/LibWeb/Forward.h
index 19ce1eae141..69ff65e3c1b 100644
--- a/Libraries/LibWeb/Forward.h
+++ b/Libraries/LibWeb/Forward.h
@@ -242,6 +242,7 @@ class CSSKeywordValue;
 class CSSLayerBlockRule;
 class CSSLayerStatementRule;
 class CSSMarginRule;
+class CSSMathNegate;
 class CSSMathProduct;
 class CSSMathSum;
 class CSSMathValue;
diff --git a/Libraries/LibWeb/idl_files.cmake b/Libraries/LibWeb/idl_files.cmake
index 33a3fadbad9..0ba14faa6c0 100644
--- a/Libraries/LibWeb/idl_files.cmake
+++ b/Libraries/LibWeb/idl_files.cmake
@@ -36,6 +36,7 @@ libweb_js_bindings(CSS/CSSKeywordValue)
 libweb_js_bindings(CSS/CSSLayerBlockRule)
 libweb_js_bindings(CSS/CSSLayerStatementRule)
 libweb_js_bindings(CSS/CSSMarginRule)
+libweb_js_bindings(CSS/CSSMathNegate)
 libweb_js_bindings(CSS/CSSMathProduct)
 libweb_js_bindings(CSS/CSSMathSum)
 libweb_js_bindings(CSS/CSSMathValue)
diff --git a/Tests/LibWeb/Text/expected/all-window-properties.txt b/Tests/LibWeb/Text/expected/all-window-properties.txt
index 0e89b57248c..95fb9230bf1 100644
--- a/Tests/LibWeb/Text/expected/all-window-properties.txt
+++ b/Tests/LibWeb/Text/expected/all-window-properties.txt
@@ -49,6 +49,7 @@ CSSKeywordValue
 CSSLayerBlockRule
 CSSLayerStatementRule
 CSSMarginRule
+CSSMathNegate
 CSSMathProduct
 CSSMathSum
 CSSMathValue