diff --git a/Libraries/LibWeb/CMakeLists.txt b/Libraries/LibWeb/CMakeLists.txt
index b181cc8bd8e..2e57ec81143 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/CSSMathValue.cpp
     CSS/CSSMediaRule.cpp
     CSS/CSSNamespaceRule.cpp
     CSS/CSSNestedDeclarations.cpp
diff --git a/Libraries/LibWeb/CSS/CSSMathValue.cpp b/Libraries/LibWeb/CSS/CSSMathValue.cpp
new file mode 100644
index 00000000000..edf39f39ec5
--- /dev/null
+++ b/Libraries/LibWeb/CSS/CSSMathValue.cpp
@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CSSMathValue.h"
+#include <LibWeb/Bindings/Intrinsics.h>
+
+namespace Web::CSS {
+
+GC_DEFINE_ALLOCATOR(CSSMathValue);
+
+CSSMathValue::CSSMathValue(JS::Realm& realm, Bindings::CSSMathOperator operator_, NumericType type)
+    : CSSNumericValue(realm, move(type))
+    , m_operator(operator_)
+{
+}
+
+void CSSMathValue::initialize(JS::Realm& realm)
+{
+    WEB_SET_PROTOTYPE_FOR_INTERFACE(CSSMathValue);
+    Base::initialize(realm);
+}
+
+}
diff --git a/Libraries/LibWeb/CSS/CSSMathValue.h b/Libraries/LibWeb/CSS/CSSMathValue.h
new file mode 100644
index 00000000000..0ea9e1da3d4
--- /dev/null
+++ b/Libraries/LibWeb/CSS/CSSMathValue.h
@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Bindings/CSSMathValuePrototype.h>
+#include <LibWeb/CSS/CSSNumericValue.h>
+
+namespace Web::CSS {
+
+// https://drafts.css-houdini.org/css-typed-om-1/#cssmathvalue
+class CSSMathValue : public CSSNumericValue {
+    WEB_PLATFORM_OBJECT(CSSMathValue, CSSNumericValue);
+    GC_DECLARE_ALLOCATOR(CSSMathValue);
+
+public:
+    virtual ~CSSMathValue() override = default;
+
+    Bindings::CSSMathOperator operator_() const { return m_operator; }
+
+    enum class Nested : u8 {
+        No,
+        Yes,
+    };
+    enum class Parens : u8 {
+        With,
+        Without,
+    };
+    virtual String serialize_math_value(Nested, Parens) const = 0;
+
+protected:
+    explicit CSSMathValue(JS::Realm&, Bindings::CSSMathOperator, NumericType);
+
+    virtual void initialize(JS::Realm&) override;
+
+    Bindings::CSSMathOperator m_operator;
+};
+
+}
diff --git a/Libraries/LibWeb/CSS/CSSMathValue.idl b/Libraries/LibWeb/CSS/CSSMathValue.idl
new file mode 100644
index 00000000000..c2fec46f20d
--- /dev/null
+++ b/Libraries/LibWeb/CSS/CSSMathValue.idl
@@ -0,0 +1,18 @@
+#import <CSS/CSSNumericValue.idl>
+
+// https://drafts.css-houdini.org/css-typed-om-1/#cssmathvalue
+[Exposed=(Window, Worker, PaintWorklet, LayoutWorklet)]
+interface CSSMathValue : CSSNumericValue {
+    readonly attribute CSSMathOperator operator;
+};
+
+// https://drafts.css-houdini.org/css-typed-om-1/#enumdef-cssmathoperator
+enum CSSMathOperator {
+    "sum",
+    "product",
+    "negate",
+    "invert",
+    "min",
+    "max",
+    "clamp",
+};
diff --git a/Libraries/LibWeb/CSS/CSSNumericValue.cpp b/Libraries/LibWeb/CSS/CSSNumericValue.cpp
index 5f240527f3f..4002af5cfff 100644
--- a/Libraries/LibWeb/CSS/CSSNumericValue.cpp
+++ b/Libraries/LibWeb/CSS/CSSNumericValue.cpp
@@ -7,6 +7,7 @@
 #include "CSSNumericValue.h"
 #include <LibWeb/Bindings/CSSNumericValuePrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/CSS/CSSMathValue.h>
 #include <LibWeb/CSS/CSSUnitValue.h>
 #include <LibWeb/CSS/NumericType.h>
 #include <LibWeb/CSS/Serialize.h>
@@ -107,9 +108,11 @@ String CSSNumericValue::to_string(SerializationParams const& params) const
     if (auto* unit_value = as_if<CSSUnitValue>(this)) {
         return unit_value->serialize_unit_value(params.minimum, params.maximum);
     }
-    // FIXME: 2. Otherwise, serialize a CSSMathValue from this, and return the result.
-
-    return {};
+    // 2. Otherwise, serialize a CSSMathValue from this, and return the result.
+    auto& math_value = as<CSSMathValue>(*this);
+    return math_value.serialize_math_value(
+        params.nested ? CSSMathValue::Nested::Yes : CSSMathValue::Nested::No,
+        params.parenless ? CSSMathValue::Parens::Without : CSSMathValue::Parens::With);
 }
 
 // https://drafts.css-houdini.org/css-typed-om-1/#rectify-a-numberish-value
diff --git a/Libraries/LibWeb/Forward.h b/Libraries/LibWeb/Forward.h
index 040d7768f06..7ae9032bdba 100644
--- a/Libraries/LibWeb/Forward.h
+++ b/Libraries/LibWeb/Forward.h
@@ -242,6 +242,7 @@ class CSSKeywordValue;
 class CSSLayerBlockRule;
 class CSSLayerStatementRule;
 class CSSMarginRule;
+class CSSMathValue;
 class CSSMediaRule;
 class CSSNamespaceRule;
 class CSSNestedDeclarations;
diff --git a/Libraries/LibWeb/idl_files.cmake b/Libraries/LibWeb/idl_files.cmake
index 19c38b5e861..3c3c817b8b5 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/CSSMathValue)
 libweb_js_bindings(CSS/CSSMediaRule)
 libweb_js_bindings(CSS/CSS NAMESPACE)
 libweb_js_bindings(CSS/CSSNamespaceRule)
diff --git a/Tests/LibWeb/Text/expected/all-window-properties.txt b/Tests/LibWeb/Text/expected/all-window-properties.txt
index 55c7f739886..347062f2e92 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
+CSSMathValue
 CSSMediaRule
 CSSNamespaceRule
 CSSNestedDeclarations