diff --git a/.changeset/validate-css-layer-name.md b/.changeset/validate-css-layer-name.md
new file mode 100644
index 00000000000..4cadc1f6f6f
--- /dev/null
+++ b/.changeset/validate-css-layer-name.md
@@ -0,0 +1,5 @@
+---
+'@clerk/ui': patch
+---
+
+Validate `appearance.cssLayerName` before wrapping component styles in `@layer`. Values that are not a valid CSS layer name (for example ones containing braces, semicolons, or markup) are now ignored with a one-time console warning instead of being interpolated into the generated stylesheet.
diff --git a/packages/ui/src/styledSystem/__tests__/createEmotionCache.test.ts b/packages/ui/src/styledSystem/__tests__/createEmotionCache.test.ts
new file mode 100644
index 00000000000..c78d064a58b
--- /dev/null
+++ b/packages/ui/src/styledSystem/__tests__/createEmotionCache.test.ts
@@ -0,0 +1,36 @@
+import { logger } from '@clerk/shared/logger';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+import { createEmotionCache } from '../createEmotionCache';
+
+function insertAndRead(cssLayerName: string | undefined, styles: string) {
+  const cache = createEmotionCache({ cssLayerName });
+  const insert = vi.spyOn(cache.sheet, 'insert').mockImplementation(() => {});
+  cache.insert('', { name: 'rule', styles, next: undefined } as any, cache.sheet, true);
+  return insert.mock.calls.map(([rule]) => rule).join('');
+}
+
+describe('createEmotionCache', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it.each(['app.clerk', '--vendor', 'clérk'])('wraps insertions in the configured layer %s', name => {
+    expect(insertAndRead(name, 'color:red;')).toContain(`@layer ${name}`);
+  });
+
+  it('drops a cssLayerName that would break out of the @layer rule', () => {
+    vi.spyOn(logger, 'warnOnce').mockImplementation(() => {});
+    const payload = 'x} body { filter: blur(2px) } /*';
+    const emitted = insertAndRead(payload, 'color:red;');
+    expect(emitted).not.toContain('@layer');
+    expect(emitted).not.toContain('blur');
+  });
+
+  it('drops a cssLayerName carrying markup', () => {
+    vi.spyOn(logger, 'warnOnce').mockImplementation(() => {});
+    const emitted = insertAndRead('x{}</style><img src=x onerror=alert(1)><style>', 'color:red;');
+    expect(emitted).not.toContain('</style>');
+    expect(emitted).not.toContain('@layer');
+  });
+});
diff --git a/packages/ui/src/styledSystem/createEmotionCache.ts b/packages/ui/src/styledSystem/createEmotionCache.ts
index 155c34942f9..fab7742f88d 100644
--- a/packages/ui/src/styledSystem/createEmotionCache.ts
+++ b/packages/ui/src/styledSystem/createEmotionCache.ts
@@ -1,6 +1,8 @@
 // eslint-disable-next-line no-restricted-imports
 import createCache, { type EmotionCache } from '@emotion/cache';
 
+import { sanitizeCssLayerName } from '../utils/cssLayerName';
+
 type CreateEmotionCacheOptions = {
   /** The nonce value for CSP (Content Security Policy). */
   nonce?: string;
@@ -14,7 +16,8 @@ type CreateEmotionCacheOptions = {
  * `cssLayerName` is set, every insertion is wrapped in `@layer <name> { ... }`
  * so consumers can control cascade precedence relative to their own styles.
  */
-export function createEmotionCache({ nonce, cssLayerName }: CreateEmotionCacheOptions): EmotionCache {
+export function createEmotionCache({ nonce, cssLayerName: rawCssLayerName }: CreateEmotionCacheOptions): EmotionCache {
+  const cssLayerName = sanitizeCssLayerName(rawCssLayerName);
   const el = typeof document !== 'undefined' ? document.querySelector('style#cl-style-insertion-point') : null;
   const cache = createCache({
     key: 'cl-internal',
diff --git a/packages/ui/src/utils/__tests__/cssLayerName.test.ts b/packages/ui/src/utils/__tests__/cssLayerName.test.ts
new file mode 100644
index 00000000000..cdf1b3418d8
--- /dev/null
+++ b/packages/ui/src/utils/__tests__/cssLayerName.test.ts
@@ -0,0 +1,123 @@
+import { logger } from '@clerk/shared/logger';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+import { isValidCssLayerName, sanitizeCssLayerName } from '../cssLayerName';
+
+// https://drafts.csswg.org/css-syntax-3/#non-ascii-ident-code-point
+const NON_ASCII_IDENT_RANGES: Array<[number, number]> = [
+  [0x00b7, 0x00b7],
+  [0x00c0, 0x00d6],
+  [0x00d8, 0x00f6],
+  [0x00f8, 0x037d],
+  [0x037f, 0x1fff],
+  [0x200c, 0x200d],
+  [0x203f, 0x2040],
+  [0x2070, 0x218f],
+  [0x2c00, 0x2fef],
+  [0x3001, 0xd7ff],
+  [0xf900, 0xfdcf],
+  [0xfdf0, 0xfffd],
+  [0x10000, 0x10ffff],
+];
+const inNonAsciiIdentRanges = (cp: number) => NON_ASCII_IDENT_RANGES.some(([lo, hi]) => cp >= lo && cp <= hi);
+const hex = (cp: number) => `U+${cp.toString(16).toUpperCase().padStart(4, '0')}`;
+const rangeEdges = [...new Set(NON_ASCII_IDENT_RANGES.flatMap(([lo, hi]) => [lo, hi]))].map(cp => ({
+  cp,
+  label: hex(cp),
+}));
+const rangeNeighbours = [...new Set(NON_ASCII_IDENT_RANGES.flatMap(([lo, hi]) => [lo - 1, hi + 1]))]
+  .filter(cp => cp <= 0x10ffff && !inNonAsciiIdentRanges(cp))
+  .map(cp => ({ cp, label: hex(cp) }));
+
+describe('cssLayerName', () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it.each([
+    'components',
+    'clerk',
+    'app.components',
+    'theme_layer-1',
+    '-vendor',
+    '--vendor',
+    '--',
+    '---',
+    '--1',
+    '_x',
+    'a.b.c',
+    'a.--b',
+    'clérk',
+    '-é',
+    'レイヤー',
+    '\u{1F600}',
+    'inherits',
+    'revert-layers',
+  ])('accepts %s', value => {
+    const warn = vi.spyOn(logger, 'warnOnce').mockImplementation(() => {});
+    expect(isValidCssLayerName(value)).toBe(true);
+    expect(sanitizeCssLayerName(value)).toBe(value);
+    expect(warn).not.toHaveBeenCalled();
+  });
+
+  it.each(rangeEdges)('accepts non-ASCII ident code point $label as start and continuation', ({ cp }) => {
+    const char = String.fromCodePoint(cp);
+    expect(isValidCssLayerName(char)).toBe(true);
+    expect(isValidCssLayerName(`a${char}`)).toBe(true);
+  });
+
+  it.each(rangeNeighbours)('rejects excluded code point $label as start and continuation', ({ cp }) => {
+    const char = String.fromCodePoint(cp);
+    expect(isValidCssLayerName(char)).toBe(false);
+    expect(isValidCssLayerName(`a${char}`)).toBe(false);
+  });
+
+  it.each([
+    'x} body { color: red } /*',
+    'x{}</style><script>alert(1)</script>',
+    'components;@import url(https://attacker.example/x)',
+    'a b',
+    'a.',
+    '.a',
+    '1abc',
+    '-1abc',
+    'a..b',
+    '',
+    ' clerk',
+    'clerk\n',
+    'a\\}b',
+    'a\u00A0b',
+    'a\u2028b',
+    'a\u00D7b',
+  ])('rejects %j', value => {
+    expect(isValidCssLayerName(value)).toBe(false);
+  });
+
+  it.each([
+    'initial',
+    'inherit',
+    'unset',
+    'revert',
+    'revert-layer',
+    'revert-rule',
+    'INITIAL',
+    'app.revert',
+    'App.Revert-Layer',
+  ])('rejects the CSS-wide keyword %s', value => {
+    expect(isValidCssLayerName(value)).toBe(false);
+  });
+
+  it('returns undefined and warns once for an invalid name', () => {
+    const warn = vi.spyOn(logger, 'warnOnce').mockImplementation(() => {});
+    expect(sanitizeCssLayerName('x} body { color: red } /*')).toBeUndefined();
+    expect(warn).toHaveBeenCalledTimes(1);
+    expect(warn.mock.calls[0][0]).toContain('cssLayerName');
+  });
+
+  it('returns undefined without warning for an empty value', () => {
+    const warn = vi.spyOn(logger, 'warnOnce').mockImplementation(() => {});
+    expect(sanitizeCssLayerName(undefined)).toBeUndefined();
+    expect(sanitizeCssLayerName('')).toBeUndefined();
+    expect(warn).not.toHaveBeenCalled();
+  });
+});
diff --git a/packages/ui/src/utils/cssLayerName.ts b/packages/ui/src/utils/cssLayerName.ts
new file mode 100644
index 00000000000..1b86d5fc9f2
--- /dev/null
+++ b/packages/ui/src/utils/cssLayerName.ts
@@ -0,0 +1,32 @@
+import { logger } from '@clerk/shared/logger';
+
+// <layer-name> = <ident> [ '.' <ident> ]*, CSS-wide keywords reserved:
+// https://drafts.csswg.org/css-cascade-5/#layer-names
+// <ident> per https://drafts.csswg.org/css-syntax-3/#ident-token-diagram, minus escape sequences.
+const NON_ASCII_IDENT =
+  '\\u00B7\\u00C0-\\u00D6\\u00D8-\\u00F6\\u00F8-\\u037D\\u037F-\\u1FFF\\u200C-\\u200D\\u203F-\\u2040\\u2070-\\u218F\\u2C00-\\u2FEF\\u3001-\\uD7FF\\uF900-\\uFDCF\\uFDF0-\\uFFFD\\u{10000}-\\u{10FFFF}';
+const IDENT_START = `[A-Za-z_${NON_ASCII_IDENT}]`;
+const IDENT_CHAR = `[A-Za-z0-9_\\-${NON_ASCII_IDENT}]`;
+const CSS_IDENT_RE = new RegExp(`^(?:--|-?${IDENT_START})${IDENT_CHAR}*$`, 'u');
+// https://drafts.csswg.org/css-cascade-5/#defaulting-keywords
+const CSS_WIDE_KEYWORDS = new Set(['initial', 'inherit', 'unset', 'revert', 'revert-layer', 'revert-rule']);
+
+export function isValidCssLayerName(value: unknown): value is string {
+  return (
+    typeof value === 'string' &&
+    value.split('.').every(segment => CSS_IDENT_RE.test(segment) && !CSS_WIDE_KEYWORDS.has(segment.toLowerCase()))
+  );
+}
+
+export function sanitizeCssLayerName(value: string | undefined): string | undefined {
+  if (!value) {
+    return undefined;
+  }
+  if (isValidCssLayerName(value)) {
+    return value;
+  }
+  logger.warnOnce(
+    `Clerk: ignoring invalid \`cssLayerName\` ${JSON.stringify(value)}. It must be a CSS layer name such as "clerk" or "app.components".`,
+  );
+  return undefined;
+}