diff --git a/.github/workflows/validate.yml b/.github/workflows/validate.yml
index c2e20a61..4a20e2ab 100644
--- a/.github/workflows/validate.yml
+++ b/.github/workflows/validate.yml
@@ -30,7 +30,7 @@ jobs:
       fail-fast: false
       matrix:
         node: [14, 16, 18]
-        react: [latest, canary, experimental]
+        react: ['18.x', latest, canary, experimental]
     runs-on: ubuntu-latest
     steps:
       - name: ⬇️ Checkout repo
diff --git a/jest.config.js b/jest.config.js
index 30654cdb..860358cd 100644
--- a/jest.config.js
+++ b/jest.config.js
@@ -3,14 +3,17 @@ const {jest: jestConfig} = require('kcd-scripts/config')
 module.exports = Object.assign(jestConfig, {
   coverageThreshold: {
     ...jestConfig.coverageThreshold,
-    // Full coverage across the build matrix (React versions) but not in a single job
+    // Full coverage across the build matrix (React 18, 19) but not in a single job
     // Ful coverage is checked via codecov
-    './src/pure.js': {
-      // minimum coverage of jobs using different React versions
-      branches: 97,
+    './src/act-compat': {
+      branches: 90,
+    },
+    './src/pure': {
+      // minimum coverage of jobs using React 18 and 19
+      branches: 95,
       functions: 88,
-      lines: 94,
-      statements: 94,
+      lines: 92,
+      statements: 92,
     },
   },
 })
diff --git a/src/__tests__/new-act.js b/src/__tests__/new-act.js
index 0412a8a3..0464ad24 100644
--- a/src/__tests__/new-act.js
+++ b/src/__tests__/new-act.js
@@ -1,10 +1,13 @@
 let asyncAct
 
-jest.mock('react-dom/test-utils', () => ({
-  act: cb => {
-    return cb()
-  },
-}))
+jest.mock('react', () => {
+  return {
+    ...jest.requireActual('react'),
+    act: cb => {
+      return cb()
+    },
+  }
+})
 
 beforeEach(() => {
   jest.resetModules()
diff --git a/src/__tests__/render.js b/src/__tests__/render.js
index 175174ca..16f7dbe2 100644
--- a/src/__tests__/render.js
+++ b/src/__tests__/render.js
@@ -3,12 +3,11 @@ import ReactDOM from 'react-dom'
 import ReactDOMServer from 'react-dom/server'
 import {fireEvent, render, screen, configure} from '../'
 
-// Needs to be changed to 19.0.0 once alpha started.
-const isReactExperimental = React.version.startsWith('18.3.0-experimental')
-const isReactCanary = React.version.startsWith('18.3.0-canary')
+const isReact18 = React.version.startsWith('18.')
+const isReact19 = React.version.startsWith('19.')
 
-// Needs to be changed to isReactExperimental || isReactCanary once alpha started.
-const testGateReact18 = isReactExperimental ? test.skip : test
+const testGateReact18 = isReact18 ? test : test.skip
+const testGateReact19 = isReact19 ? test : test.skip
 
 describe('render API', () => {
   let originalConfig
@@ -224,17 +223,21 @@ describe('render API', () => {
     expect(() => {
       render(<div />, {legacyRoot: true})
     }).toErrorDev(
-      isReactCanary
-        ? [
-            "Warning: ReactDOM.render is no longer supported in React 18. Use createRoot instead. Until you switch to the new API, your app will behave as if it's running React 17. Learn more: https://react.dev/link/switch-to-createroot",
-          ]
-        : [
-            "Warning: ReactDOM.render is no longer supported in React 18. Use createRoot instead. Until you switch to the new API, your app will behave as if it's running React 17. Learn more: https://reactjs.org/link/switch-to-createroot",
-          ],
+      [
+        "Warning: ReactDOM.render is no longer supported in React 18. Use createRoot instead. Until you switch to the new API, your app will behave as if it's running React 17. Learn more: https://reactjs.org/link/switch-to-createroot",
+      ],
       {withoutStack: true},
     )
   })
 
+  testGateReact19('legacyRoot throws', () => {
+    expect(() => {
+      render(<div />, {legacyRoot: true})
+    }).toThrowErrorMatchingInlineSnapshot(
+      `\`legacyRoot: true\` is not supported in this version of React. Please use React 18 instead.`,
+    )
+  })
+
   testGateReact18('legacyRoot uses legacy ReactDOM.hydrate', () => {
     const ui = <div />
     const container = document.createElement('div')
@@ -242,14 +245,21 @@ describe('render API', () => {
     expect(() => {
       render(ui, {container, hydrate: true, legacyRoot: true})
     }).toErrorDev(
-      isReactCanary
-        ? [
-            "Warning: ReactDOM.hydrate is no longer supported in React 18. Use hydrateRoot instead. Until you switch to the new API, your app will behave as if it's running React 17. Learn more: https://react.dev/link/switch-to-createroot",
-          ]
-        : [
-            "Warning: ReactDOM.hydrate is no longer supported in React 18. Use hydrateRoot instead. Until you switch to the new API, your app will behave as if it's running React 17. Learn more: https://reactjs.org/link/switch-to-createroot",
-          ],
+      [
+        "Warning: ReactDOM.hydrate is no longer supported in React 18. Use hydrateRoot instead. Until you switch to the new API, your app will behave as if it's running React 17. Learn more: https://reactjs.org/link/switch-to-createroot",
+      ],
       {withoutStack: true},
     )
   })
+
+  testGateReact19('legacyRoot throws even with hydrate', () => {
+    const ui = <div />
+    const container = document.createElement('div')
+    container.innerHTML = ReactDOMServer.renderToString(ui)
+    expect(() => {
+      render(ui, {container, hydrate: true, legacyRoot: true})
+    }).toThrowErrorMatchingInlineSnapshot(
+      `\`legacyRoot: true\` is not supported in this version of React. Please use React 18 instead.`,
+    )
+  })
 })
diff --git a/src/__tests__/renderHook.js b/src/__tests__/renderHook.js
index 34259b44..c7c8b066 100644
--- a/src/__tests__/renderHook.js
+++ b/src/__tests__/renderHook.js
@@ -1,12 +1,11 @@
 import React from 'react'
 import {renderHook} from '../pure'
 
-// Needs to be changed to 19.0.0 once alpha started.
-const isReactExperimental = React.version.startsWith('18.3.0-experimental')
-const isReactCanary = React.version.startsWith('18.3.0')
+const isReact18 = React.version.startsWith('18.')
+const isReact19 = React.version.startsWith('19.')
 
-// Needs to be changed to isReactExperimental || isReactCanary once alpha started.
-const testGateReact18 = isReactExperimental ? test.skip : test
+const testGateReact18 = isReact18 ? test : test.skip
+const testGateReact19 = isReact19 ? test : test.skip
 
 test('gives committed result', () => {
   const {result} = renderHook(() => {
@@ -85,14 +84,30 @@ testGateReact18('legacyRoot uses legacy ReactDOM.render', () => {
       },
     ).result
   }).toErrorDev(
-    isReactCanary
-      ? [
-          "Warning: ReactDOM.render is no longer supported in React 18. Use createRoot instead. Until you switch to the new API, your app will behave as if it's running React 17. Learn more: https://react.dev/link/switch-to-createroot",
-        ]
-      : [
-          "Warning: ReactDOM.render is no longer supported in React 18. Use createRoot instead. Until you switch to the new API, your app will behave as if it's running React 17. Learn more: https://reactjs.org/link/switch-to-createroot",
-        ],
+    [
+      "Warning: ReactDOM.render is no longer supported in React 18. Use createRoot instead. Until you switch to the new API, your app will behave as if it's running React 17. Learn more: https://reactjs.org/link/switch-to-createroot",
+    ],
     {withoutStack: true},
   )
   expect(result.current).toEqual('provided')
 })
+
+testGateReact19('legacyRoot throws', () => {
+  const Context = React.createContext('default')
+  function Wrapper({children}) {
+    return <Context.Provider value="provided">{children}</Context.Provider>
+  }
+  expect(() => {
+    renderHook(
+      () => {
+        return React.useContext(Context)
+      },
+      {
+        wrapper: Wrapper,
+        legacyRoot: true,
+      },
+    ).result
+  }).toThrowErrorMatchingInlineSnapshot(
+    `\`legacyRoot: true\` is not supported in this version of React. Please use React 18 instead.`,
+  )
+})
diff --git a/src/act-compat.js b/src/act-compat.js
index 86518196..5877755c 100644
--- a/src/act-compat.js
+++ b/src/act-compat.js
@@ -1,6 +1,7 @@
-import * as testUtils from 'react-dom/test-utils'
+import * as React from 'react'
+import * as DeprecatedReactTestUtils from 'react-dom/test-utils'
 
-const domAct = testUtils.act
+const reactAct = React.act ?? DeprecatedReactTestUtils.act
 
 function getGlobalThis() {
   /* istanbul ignore else */
@@ -78,7 +79,7 @@ function withGlobalActEnvironment(actImplementation) {
   }
 }
 
-const act = withGlobalActEnvironment(domAct)
+const act = withGlobalActEnvironment(reactAct)
 
 export default act
 export {
diff --git a/src/pure.js b/src/pure.js
index 3939a11a..38ec519f 100644
--- a/src/pure.js
+++ b/src/pure.js
@@ -207,6 +207,14 @@ function render(
     wrapper,
   } = {},
 ) {
+  if (legacyRoot && typeof ReactDOM.render !== 'function') {
+    const error = new Error(
+      '`legacyRoot: true` is not supported in this version of React. Please use React 18 instead.',
+    )
+    Error.captureStackTrace(error, render)
+    throw error
+  }
+
   if (!baseElement) {
     // default to document.body instead of documentElement to avoid output of potentially-large
     // head elements (such as JSS style blocks) in debug output
@@ -263,6 +271,15 @@ function cleanup() {
 
 function renderHook(renderCallback, options = {}) {
   const {initialProps, ...renderOptions} = options
+
+  if (renderOptions.legacyRoot && typeof ReactDOM.render !== 'function') {
+    const error = new Error(
+      '`legacyRoot: true` is not supported in this version of React. Please use React 18 instead.',
+    )
+    Error.captureStackTrace(error, renderHook)
+    throw error
+  }
+
   const result = React.createRef()
 
   function TestComponent({renderCallbackProps}) {
diff --git a/types/index.d.ts b/types/index.d.ts
index 49a1b7ff..e7cf02bc 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -73,6 +73,7 @@ export interface RenderOptions<
    */
   hydrate?: boolean
   /**
+   * Only works if used with React 18.
    * Set to `true` if you want to force synchronous `ReactDOM.render`.
    * Otherwise `render` will default to concurrent React if available.
    */