diff --git a/change/@fluentui-react-provider-6afbfd01-d257-41ee-99b8-bfe470e89675.json b/change/@fluentui-react-provider-6afbfd01-d257-41ee-99b8-bfe470e89675.json
new file mode 100644
index 00000000000000..a5054dc5b3244c
--- /dev/null
+++ b/change/@fluentui-react-provider-6afbfd01-d257-41ee-99b8-bfe470e89675.json
@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "fix: prevent XSS theme vulnerability during SSR",
+  "packageName": "@fluentui/react-provider",
+  "email": "martinhochel@microsoft.com",
+  "dependentChangeType": "patch"
+}
diff --git a/packages/react-components/react-provider/library/src/components/FluentProvider/createCSSRuleFromTheme.test.ts b/packages/react-components/react-provider/library/src/components/FluentProvider/createCSSRuleFromTheme.test.ts
index 11a34ad40c364b..37649092f68609 100644
--- a/packages/react-components/react-provider/library/src/components/FluentProvider/createCSSRuleFromTheme.test.ts
+++ b/packages/react-components/react-provider/library/src/components/FluentProvider/createCSSRuleFromTheme.test.ts
@@ -16,4 +16,17 @@ describe('createCSSRuleFromTheme', () => {
       `".selector { --borderRadiusLarge: 10px; --colorBackgroundOverlay: rgba(0, 0, 0, 0.4);  }"`,
     );
   });
+
+  it('prevents XSS by replacing angle brackets that could inject HTML', () => {
+    const theme = {
+      colorBrandBackground: '</style><script>alert("xss")</script>',
+    } as PartialTheme;
+
+    const result = createCSSRuleFromTheme('.selector', theme);
+    expect(result).not.toContain('<');
+    expect(result).not.toContain('>');
+    expect(result).toMatchInlineSnapshot(
+      `".selector { --colorBrandBackground: \\\\3C /style\\\\3E \\\\3C script\\\\3E alert(\\"xss\\")\\\\3C /script\\\\3E ;  }"`,
+    );
+  });
 });
diff --git a/packages/react-components/react-provider/library/src/components/FluentProvider/createCSSRuleFromTheme.ts b/packages/react-components/react-provider/library/src/components/FluentProvider/createCSSRuleFromTheme.ts
index 043edc46b58159..41a15a91ae8914 100644
--- a/packages/react-components/react-provider/library/src/components/FluentProvider/createCSSRuleFromTheme.ts
+++ b/packages/react-components/react-provider/library/src/components/FluentProvider/createCSSRuleFromTheme.ts
@@ -1,5 +1,21 @@
 import type { PartialTheme } from '@fluentui/react-theme';
 
+const CSS_ESCAPE_MAP = {
+  '<': '\\3C ',
+  '>': '\\3E ',
+};
+/**
+ * Escapes characters that could break out of a <style> tag during SSR.
+ *
+ * IMPORTANT: Do not strip quotes. Theme values legitimately include quoted font families and other CSS.
+ * We only need to ensure the generated text cannot terminate the style tag and inject HTML.
+ */
+function escapeForStyleTag(value: string): string {
+  // Escape as CSS code points so the resulting CSS still represents the same characters.
+  // Using CSS escapes prevents the HTML parser from seeing a literal '<' / '>' and closing <style>.
+  return value.replace(/[<>]/g, match => CSS_ESCAPE_MAP[match as keyof typeof CSS_ESCAPE_MAP]);
+}
+
 /**
  * Creates a CSS rule from a theme object.
  *
@@ -11,7 +27,7 @@ export function createCSSRuleFromTheme(selector: string, theme: PartialTheme | u
       return `${cssVarRule}--${cssVar}: ${theme[cssVar]}; `;
     }, '');
 
-    return `${selector} { ${cssVarsAsString} }`;
+    return `${selector} { ${escapeForStyleTag(cssVarsAsString)} }`;
   }
 
   return `${selector} {}`;