diff --git a/src/pseudo-selectors/filters.ts b/src/pseudo-selectors/filters.ts
index 6fe37384..42ac0678 100644
--- a/src/pseudo-selectors/filters.ts
+++ b/src/pseudo-selectors/filters.ts
@@ -4,6 +4,31 @@ import { cacheParentResults } from "../helpers/cache.js";
 import { getElementParent } from "../helpers/querying.js";
 import type { CompiledQuery, InternalOptions } from "../types.js";
 
+/**
+ * RFC 4647 extended filtering with pre-split subtags.
+ * @param tag - Lowercased subtags of the element's language value.
+ * @param range - Lowercased subtags of the language range to match against.
+ */
+function extendedFilter(tag: string[], range: string[]): boolean {
+    if (range[0] !== "*" && range[0] !== tag[0]) return false;
+
+    let tagIndex = 1;
+
+    for (let rangeIndex = 1; rangeIndex < range.length; rangeIndex++) {
+        if (range[rangeIndex] === "*") continue;
+
+        // Skip non-singleton tag subtags until we find a match.
+        while (tagIndex < tag.length && tag[tagIndex] !== range[rangeIndex]) {
+            if (tag[tagIndex++].length <= 1) return false;
+        }
+
+        if (tagIndex >= tag.length) return false;
+        tagIndex++;
+    }
+
+    return true;
+}
+
 type Filter = <Node, ElementNode extends Node>(
     next: CompiledQuery<ElementNode>,
     text: string,
@@ -175,6 +200,49 @@ export const filters: Record<string, Filter> = {
         return (element) => context.includes(element) && next(element);
     },
 
+    lang(next, code, { adapter }) {
+        const ranges = code
+            .split(",")
+            .map((r) => r.trim())
+            .filter((r) => r.length > 0)
+            .map((r) =>
+                r
+                    .replace(/^['"]|['"]$/g, "")
+                    .toLowerCase()
+                    .split("-"),
+            );
+
+        return function lang(element) {
+            let node: typeof element | null = element;
+
+            while (node != null) {
+                const value =
+                    adapter.getAttributeValue(node, "xml:lang") ??
+                    adapter.getAttributeValue(node, "lang");
+
+                if (value != null) {
+                    if (!value) {
+                        return ranges.some((r) => r[0] === "") && next(element);
+                    }
+
+                    const tag = value.toLowerCase().split("-");
+                    return (
+                        ranges.some((r) => extendedFilter(tag, r)) &&
+                        next(element)
+                    );
+                }
+
+                const parent = adapter.getParent(node);
+                node =
+                    parent != null && adapter.isTag(parent)
+                        ? (parent as typeof element)
+                        : null;
+            }
+
+            return ranges.some((r) => r[0] === "") && next(element);
+        };
+    },
+
     hover: dynamicStatePseudo("isHovered"),
     visited: dynamicStatePseudo("isVisited"),
     active: dynamicStatePseudo("isActive"),
diff --git a/test/pseudo-classes.ts b/test/pseudo-classes.ts
index 316ad1de..c1e70929 100644
--- a/test/pseudo-classes.ts
+++ b/test/pseudo-classes.ts
@@ -206,6 +206,64 @@ describe(":has", () => {
     });
 });
 
+describe(":lang", () => {
+    // Single fixture covering inheritance, override, and untagged elements.
+    const langFixture = parseDocument(
+        '<div lang="en"><p id="a">A</p><div lang="fr-BE"><p id="b">B</p></div></div><p id="c">C</p>',
+    );
+
+    it.each([
+        // [selector, expected ids]
+        [":lang(en)", ["a"]],
+        [":lang(EN)", ["a"]],
+        [":lang(fr)", ["b"]],
+        [":lang(fr-BE)", ["b"]],
+        [":lang(en, fr)", ["a", "b"]],
+        [":lang(de)", []],
+    ])("%s matches %j", (selector, expectedIds) => {
+        const matches = CSSselect.selectAll<AnyNode, Element>(
+            `p${selector}`,
+            langFixture,
+        );
+        expect(matches.map((element) => element.attribs["id"])).toStrictEqual(
+            expectedIds,
+        );
+    });
+
+    it("should not match untagged elements", () => {
+        expect(
+            CSSselect.selectAll<AnyNode, Element>("p:lang(en)", langFixture),
+        ).toHaveLength(1);
+    });
+
+    it("should use extended filtering", () => {
+        const dom = parseDocument(
+            '<p lang="de-DE">a</p><p lang="de-Latn-DE">b</p><p lang="de-Latn-DE-1996">c</p>',
+        );
+        expect(
+            CSSselect.selectAll<AnyNode, Element>(":lang(de-DE)", dom),
+        ).toHaveLength(3);
+    });
+
+    it("should support wildcard primary subtag", () => {
+        const dom = parseDocument(
+            '<p lang="de-CH">a</p><p lang="fr-CH">b</p><p lang="fr-FR">c</p>',
+        );
+        expect(
+            CSSselect.selectAll<AnyNode, Element>(":lang(\\*-CH)", dom),
+        ).toHaveLength(2);
+    });
+
+    it("should support xml:lang", () => {
+        const dom = parseDocument('<div xml:lang="ja"><p>x</p></div>', {
+            xmlMode: true,
+        });
+        expect(
+            CSSselect.selectAll<AnyNode, Element>(":lang(ja)", dom),
+        ).toHaveLength(2);
+    });
+});
+
 describe(":read-only and :read-write", () => {
     it("should match", () => {
         const dom = parseDocument(`