diff --git a/.changeset/fix-inert-react-19-compat.md b/.changeset/fix-inert-react-19-compat.md
new file mode 100644
index 00000000000..2068744f5a9
--- /dev/null
+++ b/.changeset/fix-inert-react-19-compat.md
@@ -0,0 +1,5 @@
+---
+"@clerk/ui": patch
+---
+
+Fix `inert` attribute to work correctly in React 19. The previous value `''` (empty string) is falsy and not set by React 19's boolean attribute handler, leaving hidden panel and collapsible content interactive. Switches to `'true'` which is truthy in both React 18 and 19.
diff --git a/packages/headless/src/global.d.ts b/packages/headless/src/global.d.ts
new file mode 100644
index 00000000000..34c921452ae
--- /dev/null
+++ b/packages/headless/src/global.d.ts
@@ -0,0 +1,13 @@
+// `export {}` makes this file a module so `declare module 'react'` augments React's
+// types instead of replacing them (an ambient declaration would shadow React, breaking
+// type resolution package-wide).
+export {};
+
+declare module 'react' {
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  interface HTMLAttributes<T> {
+    // `inert` landed in @types/react v19; augment for React 18 compatibility.
+    // Use 'true' (truthy string) — '' is falsy in React 19 and won't set the attribute.
+    inert?: 'true' | undefined;
+  }
+}
diff --git a/packages/headless/src/primitives/tabs/tabs.test.tsx b/packages/headless/src/primitives/tabs/tabs.test.tsx
index 24a8ec960dc..ca344c0107c 100644
--- a/packages/headless/src/primitives/tabs/tabs.test.tsx
+++ b/packages/headless/src/primitives/tabs/tabs.test.tsx
@@ -506,6 +506,29 @@ describe('Tabs', () => {
       const panels = document.querySelectorAll('[data-cl-slot="tabs-panel"][data-cl-hidden]');
       expect(panels).toHaveLength(2);
     });
+
+    it('non-selected panels have inert attribute, selected panel does not', () => {
+      renderTabs();
+      const panels = document.querySelectorAll('[data-cl-slot="tabs-panel"]');
+      const inert = Array.from(panels).filter(p => p.hasAttribute('inert'));
+      const notInert = Array.from(panels).filter(p => !p.hasAttribute('inert'));
+      // Presence check only — React 18 renders inert="true", React 19 normalises to inert=""
+      expect(inert).toHaveLength(2);
+      expect(notInert).toHaveLength(1);
+    });
+
+    it('inert updates when selection changes', async () => {
+      const user = userEvent.setup();
+      renderTabs();
+
+      await user.click(screen.getByText('Settings'));
+
+      const panels = document.querySelectorAll('[data-cl-slot="tabs-panel"]');
+      const [account, settings, billing] = Array.from(panels);
+      expect(account).toHaveAttribute('inert');
+      expect(settings).not.toHaveAttribute('inert');
+      expect(billing).toHaveAttribute('inert');
+    });
   });
 
   describe('roving tabindex', () => {
diff --git a/packages/ui/src/components/PricingTable/PricingTableMatrix.tsx b/packages/ui/src/components/PricingTable/PricingTableMatrix.tsx
index e6ab609343b..4c17b858aac 100644
--- a/packages/ui/src/components/PricingTable/PricingTableMatrix.tsx
+++ b/packages/ui/src/components/PricingTable/PricingTableMatrix.tsx
@@ -265,7 +265,6 @@ export function PricingTableMatrix({
                                   }),
                                   feePeriodNoticeAnimation,
                                 ]}
-                                // @ts-ignore - Needed until React 19 support
                                 inert={planPeriod !== 'annual' ? 'true' : undefined}
                               >
                                 <Box
diff --git a/packages/ui/src/components/devPrompts/KeylessPrompt/index.tsx b/packages/ui/src/components/devPrompts/KeylessPrompt/index.tsx
index e9e6854c03c..ecd2f78d92f 100644
--- a/packages/ui/src/components/devPrompts/KeylessPrompt/index.tsx
+++ b/packages/ui/src/components/devPrompts/KeylessPrompt/index.tsx
@@ -506,7 +506,7 @@ function KeylessPromptInternal(props: KeylessPromptProps) {
       </button>
       <div
         id={id}
-        {...(!isOpen && { inert: '' as any })}
+        {...(!isOpen && { inert: 'true' })}
         css={css`
           ${CSS_RESET};
           display: grid;
diff --git a/packages/ui/src/elements/Collapsible.tsx b/packages/ui/src/elements/Collapsible.tsx
index 8430ecd0333..c689a280909 100644
--- a/packages/ui/src/elements/Collapsible.tsx
+++ b/packages/ui/src/elements/Collapsible.tsx
@@ -75,8 +75,7 @@ export function Collapsible({ open, children, sx }: CollapsibleProps): JSX.Eleme
         }),
         sx,
       ]}
-      // @ts-ignore - inert not yet in React types
-      inert={!open ? '' : undefined}
+      inert={!open ? 'true' : undefined}
     >
       <Box
         elementDescriptor={descriptors.collapsibleInner}
diff --git a/packages/ui/src/elements/__tests__/Collapsible.test.tsx b/packages/ui/src/elements/__tests__/Collapsible.test.tsx
index 2a7e89f0643..e0076a783c4 100644
--- a/packages/ui/src/elements/__tests__/Collapsible.test.tsx
+++ b/packages/ui/src/elements/__tests__/Collapsible.test.tsx
@@ -367,7 +367,7 @@ describe('Collapsible', () => {
   });
 
   describe('Inert Attribute', () => {
-    it('sets inert to empty string when open={false}', async () => {
+    it('sets inert when open={false}', async () => {
       const { wrapper } = await createFixtures();
       const { container, rerender } = render(<Collapsible open>Content</Collapsible>, { wrapper });
 
@@ -375,7 +375,8 @@ describe('Collapsible', () => {
       rerender(<Collapsible open={false}>Content</Collapsible>);
 
       const element = container.querySelector('.cl-collapsible') as HTMLElement;
-      expect(element).toHaveAttribute('inert', '');
+      // Check presence only — React 18 renders inert="true", React 19 normalises to inert=""
+      expect(element).toHaveAttribute('inert');
     });
 
     it('does not set inert when open={true}', async () => {
diff --git a/packages/ui/src/global.d.ts b/packages/ui/src/global.d.ts
index 7bbffaeda66..19db755dffa 100644
--- a/packages/ui/src/global.d.ts
+++ b/packages/ui/src/global.d.ts
@@ -1,6 +1,14 @@
 import type { Clerk } from '@clerk/shared/types';
 import type { ClerkUIConstructor } from '@clerk/shared/ui';
 
+declare module 'react' {
+  interface HTMLAttributes<T> {
+    // `inert` landed in @types/react v19; augment for React 18 compatibility.
+    // Use 'true' (truthy string) — '' is falsy in React 19 and won't set the attribute.
+    inert?: 'true' | undefined;
+  }
+}
+
 declare module '*.svg' {
   const value: React.FC<React.SVGAttributes<SVGElement>>;
   export default value;