diff --git a/apps/frontend/src/shared/state-machines/index.ts b/apps/frontend/src/shared/state-machines/index.ts
index 68cd1f3a..1c9a29fc 100644
--- a/apps/frontend/src/shared/state-machines/index.ts
+++ b/apps/frontend/src/shared/state-machines/index.ts
@@ -7,3 +7,12 @@ export {
   mapStateToLegacy,
 } from './task-state-utils';
 export type { TaskStateName } from './task-state-utils';
+
+export { prReviewMachine } from './pr-review-machine';
+export type { PRReviewContext, PRReviewEvent } from './pr-review-machine';
+export {
+  PR_REVIEW_STATE_NAMES,
+  PR_REVIEW_SETTLED_STATES,
+  mapPRReviewStateToLegacy,
+} from './pr-review-state-utils';
+export type { PRReviewStateName } from './pr-review-state-utils';
diff --git a/apps/frontend/src/shared/state-machines/pr-review-machine.ts b/apps/frontend/src/shared/state-machines/pr-review-machine.ts
new file mode 100644
index 00000000..fd14698a
--- /dev/null
+++ b/apps/frontend/src/shared/state-machines/pr-review-machine.ts
@@ -0,0 +1,177 @@
+import { assign, createMachine } from 'xstate';
+import type { PRReviewProgress, PRReviewResult } from '../../preload/api/modules/github-api';
+
+export interface PRReviewContext {
+  prNumber: number | null;
+  projectId: string | null;
+  startedAt: string | null;
+  isFollowup: boolean;
+  progress: PRReviewProgress | null;
+  result: PRReviewResult | null;
+  previousResult: PRReviewResult | null;
+  error: string | null;
+  isExternalReview: boolean;
+}
+
+export type PRReviewEvent =
+  | { type: 'START_REVIEW'; prNumber: number; projectId: string }
+  | { type: 'START_FOLLOWUP_REVIEW'; prNumber: number; projectId: string; previousResult: PRReviewResult }
+  | { type: 'SET_PROGRESS'; progress: PRReviewProgress }
+  | { type: 'REVIEW_COMPLETE'; result: PRReviewResult }
+  | { type: 'REVIEW_ERROR'; error: string }
+  | { type: 'CANCEL_REVIEW' }
+  | { type: 'DETECT_EXTERNAL_REVIEW' }
+  | { type: 'CLEAR_REVIEW' };
+
+const initialContext: PRReviewContext = {
+  prNumber: null,
+  projectId: null,
+  startedAt: null,
+  isFollowup: false,
+  progress: null,
+  result: null,
+  previousResult: null,
+  error: null,
+  isExternalReview: false,
+};
+
+export const prReviewMachine = createMachine(
+  {
+    id: 'prReview',
+    initial: 'idle',
+    types: {} as {
+      context: PRReviewContext;
+      events: PRReviewEvent;
+    },
+    context: { ...initialContext },
+    states: {
+      idle: {
+        on: {
+          START_REVIEW: {
+            target: 'reviewing',
+            actions: 'setReviewStart',
+          },
+          START_FOLLOWUP_REVIEW: {
+            target: 'reviewing',
+            actions: 'setFollowupReviewStart',
+          },
+        },
+      },
+      reviewing: {
+        on: {
+          START_REVIEW: {
+            target: 'reviewing',
+            guard: 'isNotAlreadyReviewing',
+          },
+          SET_PROGRESS: {
+            actions: 'setProgress',
+          },
+          REVIEW_COMPLETE: {
+            target: 'completed',
+            actions: 'setResult',
+          },
+          REVIEW_ERROR: {
+            target: 'error',
+            actions: 'setError',
+          },
+          CANCEL_REVIEW: {
+            target: 'error',
+            actions: 'setCancelledError',
+          },
+          DETECT_EXTERNAL_REVIEW: {
+            target: 'externalReview',
+            actions: 'setExternalReview',
+          },
+        },
+      },
+      externalReview: {
+        on: {
+          REVIEW_COMPLETE: {
+            target: 'completed',
+            actions: 'setResult',
+          },
+          REVIEW_ERROR: {
+            target: 'error',
+            actions: 'setError',
+          },
+        },
+      },
+      completed: {
+        on: {
+          START_REVIEW: {
+            target: 'reviewing',
+            actions: 'setReviewStart',
+          },
+          START_FOLLOWUP_REVIEW: {
+            target: 'reviewing',
+            actions: 'setFollowupReviewStart',
+          },
+          CLEAR_REVIEW: {
+            target: 'idle',
+            actions: 'clearContext',
+          },
+        },
+      },
+      error: {
+        on: {
+          START_REVIEW: {
+            target: 'reviewing',
+            actions: 'setReviewStart',
+          },
+          CLEAR_REVIEW: {
+            target: 'idle',
+            actions: 'clearContext',
+          },
+        },
+      },
+    },
+  },
+  {
+    guards: {
+      isNotAlreadyReviewing: () => false,
+    },
+    actions: {
+      setReviewStart: assign({
+        prNumber: ({ event }) => (event as { prNumber: number }).prNumber,
+        projectId: ({ event }) => (event as { projectId: string }).projectId,
+        startedAt: () => new Date().toISOString(),
+        isFollowup: () => false,
+        progress: () => null,
+        result: () => null,
+        previousResult: () => null,
+        error: () => null,
+        isExternalReview: () => false,
+      }),
+      setFollowupReviewStart: assign({
+        prNumber: ({ event }) => (event as { prNumber: number }).prNumber,
+        projectId: ({ event }) => (event as { projectId: string }).projectId,
+        startedAt: () => new Date().toISOString(),
+        isFollowup: () => true,
+        progress: () => null,
+        result: () => null,
+        previousResult: ({ event }) => (event as { previousResult: PRReviewResult }).previousResult,
+        error: () => null,
+        isExternalReview: () => false,
+      }),
+      setProgress: assign({
+        progress: ({ event }) => (event as { progress: PRReviewProgress }).progress,
+      }),
+      setResult: assign({
+        result: ({ event }) => (event as { result: PRReviewResult }).result,
+        progress: () => null,
+      }),
+      setError: assign({
+        error: ({ event }) => (event as { error: string }).error,
+        progress: () => null,
+      }),
+      setCancelledError: assign({
+        error: () => 'Review cancelled by user',
+        progress: () => null,
+      }),
+      setExternalReview: assign({
+        isExternalReview: () => true,
+      }),
+      clearContext: assign(() => ({ ...initialContext })),
+    },
+  }
+);
diff --git a/apps/frontend/src/shared/state-machines/pr-review-state-utils.ts b/apps/frontend/src/shared/state-machines/pr-review-state-utils.ts
new file mode 100644
index 00000000..a45f41a4
--- /dev/null
+++ b/apps/frontend/src/shared/state-machines/pr-review-state-utils.ts
@@ -0,0 +1,52 @@
+/**
+ * Shared XState PR review state utilities.
+ *
+ * Provides type-safe state names, settled states, and legacy status conversion
+ * derived from the PR review machine definition.
+ */
+
+/**
+ * All XState PR review state names.
+ *
+ * IMPORTANT: These must match the state keys in pr-review-machine.ts.
+ * If you add/remove a state in the machine, update this array.
+ */
+export const PR_REVIEW_STATE_NAMES = [
+  'idle', 'reviewing', 'externalReview', 'completed', 'error'
+] as const;
+
+export type PRReviewStateName = typeof PR_REVIEW_STATE_NAMES[number];
+
+/**
+ * XState states where the PR review has "settled" — the review lifecycle
+ * has reached a terminal or resting state. Progress events should not
+ * overwrite these states.
+ */
+export const PR_REVIEW_SETTLED_STATES: ReadonlySet<string> = new Set<PRReviewStateName>([
+  'completed', 'error'
+]);
+
+/**
+ * Legacy review status values used by the existing Zustand store.
+ */
+type LegacyPRReviewStatus = 'idle' | 'reviewing' | 'completed' | 'error';
+
+/**
+ * Convert XState PR review state to legacy status for backward compatibility.
+ */
+export function mapPRReviewStateToLegacy(state: string): LegacyPRReviewStatus {
+  switch (state) {
+    case 'idle':
+      return 'idle';
+    case 'reviewing':
+      return 'reviewing';
+    case 'externalReview':
+      return 'reviewing';
+    case 'completed':
+      return 'completed';
+    case 'error':
+      return 'error';
+    default:
+      return 'idle';
+  }
+}