diff --git a/packages/common/src/services/decision/deleteInstance.ts b/packages/common/src/services/decision/deleteInstance.ts
new file mode 100644
index 000000000..f2ac064f2
--- /dev/null
+++ b/packages/common/src/services/decision/deleteInstance.ts
@@ -0,0 +1,85 @@
+import { db, eq } from '@op/db/client';
+import { ProcessStatus, processInstances } from '@op/db/schema';
+import { User } from '@op/supabase/lib';
+import { assertAccess, permission } from 'access-zones';
+
+import { CommonError, NotFoundError } from '../../utils';
+import { getProfileAccessUser } from '../access';
+
+export interface DeleteInstanceResult {
+  success: boolean;
+  action: 'deleted' | 'cancelled';
+  instanceId: string;
+}
+
+export const deleteInstance = async ({
+  instanceId,
+  user,
+}: {
+  instanceId: string;
+  user: User;
+}): Promise<DeleteInstanceResult> => {
+  // Fetch existing instance
+  const existingInstance = await db._query.processInstances.findFirst({
+    where: (table, { eq }) => eq(table.id, instanceId),
+  });
+
+  if (!existingInstance) {
+    throw new NotFoundError('Process instance not found');
+  }
+
+  const { profileId } = existingInstance;
+  if (!profileId) {
+    throw new CommonError(
+      'Decision instance does not have an associated profile',
+    );
+  }
+
+  // Check user has admin access on the decision instance's profile and check for transitions in parallel
+  const [profileUser, transitions] = await Promise.all([
+    getProfileAccessUser({ user, profileId }),
+    db._query.stateTransitionHistory.findFirst({
+      where: (table, { eq }) => eq(table.processInstanceId, instanceId),
+    }),
+  ]);
+
+  assertAccess({ profile: permission.ADMIN }, profileUser?.roles ?? []);
+
+  if (transitions) {
+    // Transitions exist - cancel instead of delete
+    const [cancelledInstance] = await db
+      .update(processInstances)
+      .set({
+        status: ProcessStatus.CANCELLED,
+        updatedAt: new Date().toISOString(),
+      })
+      .where(eq(processInstances.id, instanceId))
+      .returning();
+
+    if (!cancelledInstance) {
+      throw new CommonError('Failed to cancel process instance');
+    }
+
+    return {
+      success: true,
+      action: 'cancelled',
+      instanceId,
+    };
+  }
+
+  // No transitions - safe to hard delete
+  const [deletedInstance] = await db
+    .delete(processInstances)
+    .where(eq(processInstances.id, instanceId))
+    .returning();
+
+  if (!deletedInstance) {
+    throw new CommonError('Failed to delete process instance');
+  }
+
+  return {
+    success: true,
+    action: 'deleted',
+    instanceId,
+  };
+};
diff --git a/packages/common/src/services/decision/index.ts b/packages/common/src/services/decision/index.ts
index 1c60ceffa..ae4eab285 100644
--- a/packages/common/src/services/decision/index.ts
+++ b/packages/common/src/services/decision/index.ts
@@ -10,6 +10,7 @@ export * from './createInstance';
 export * from './createInstanceFromTemplate';
 export * from './updateInstance';
 export * from './updateDecisionInstance';
+export * from './deleteInstance';
 export * from './listInstances';
 export * from './getInstance';
 export * from './listDecisionProfiles';
diff --git a/services/api/src/routers/decision/instances/deleteInstance.test.ts b/services/api/src/routers/decision/instances/deleteInstance.test.ts
new file mode 100644
index 000000000..64e3b58e3
--- /dev/null
+++ b/services/api/src/routers/decision/instances/deleteInstance.test.ts
@@ -0,0 +1,258 @@
+import { db, eq } from '@op/db/client';
+import {
+  ProcessStatus,
+  processInstances,
+  stateTransitionHistory,
+} from '@op/db/schema';
+import { describe, expect, it } from 'vitest';
+
+import { appRouter } from '../..';
+import { TestDecisionsDataManager } from '../../../test/helpers/TestDecisionsDataManager';
+import {
+  createIsolatedSession,
+  createTestContextWithSession,
+} from '../../../test/supabase-utils';
+import { createCallerFactory } from '../../../trpcFactory';
+
+const createCaller = createCallerFactory(appRouter);
+
+async function createAuthenticatedCaller(email: string) {
+  const { session } = await createIsolatedSession(email);
+  return createCaller(await createTestContextWithSession(session));
+}
+
+describe.concurrent('deleteInstance', () => {
+  it('should hard delete instance when no transitions exist', async ({
+    task,
+    onTestFinished,
+  }) => {
+    const testData = new TestDecisionsDataManager(task.id, onTestFinished);
+
+    const setup = await testData.createDecisionSetup({
+      instanceCount: 1,
+      grantAccess: true,
+    });
+
+    const { instance } = setup.instances[0]!;
+    const caller = await createAuthenticatedCaller(setup.userEmail);
+
+    const result = await caller.decision.deleteInstance({
+      instanceId: instance.id,
+    });
+
+    expect(result.success).toBe(true);
+    expect(result.action).toBe('deleted');
+    expect(result.instanceId).toBe(instance.id);
+
+    // Verify the instance was actually deleted
+    const deletedInstance = await db._query.processInstances.findFirst({
+      where: (table, { eq }) => eq(table.id, instance.id),
+    });
+
+    expect(deletedInstance).toBeUndefined();
+  });
+
+  it('should cancel instance when transitions exist', async ({
+    task,
+    onTestFinished,
+  }) => {
+    const testData = new TestDecisionsDataManager(task.id, onTestFinished);
+
+    const setup = await testData.createDecisionSetup({
+      instanceCount: 1,
+      grantAccess: true,
+    });
+
+    const { instance } = setup.instances[0]!;
+    const caller = await createAuthenticatedCaller(setup.userEmail);
+
+    // Insert a state transition history record to simulate a transition having occurred
+    await db.insert(stateTransitionHistory).values([
+      {
+        processInstanceId: instance.id,
+        fromStateId: 'initial',
+        toStateId: 'final',
+        transitionedAt: new Date(),
+      },
+    ]);
+
+    const result = await caller.decision.deleteInstance({
+      instanceId: instance.id,
+    });
+
+    expect(result.success).toBe(true);
+    expect(result.action).toBe('cancelled');
+    expect(result.instanceId).toBe(instance.id);
+
+    // Verify the instance was cancelled, not deleted
+    const cancelledInstance = await db._query.processInstances.findFirst({
+      where: (table, { eq }) => eq(table.id, instance.id),
+    });
+
+    expect(cancelledInstance).toBeDefined();
+    expect(cancelledInstance!.status).toBe('cancelled');
+  });
+
+  it('should require authentication', async () => {
+    const caller = createCaller({
+      session: null,
+      user: null,
+    } as never);
+
+    await expect(
+      caller.decision.deleteInstance({
+        instanceId: '00000000-0000-0000-0000-000000000000',
+      }),
+    ).rejects.toThrow(/authenticate/i);
+  });
+
+  it('should throw error for non-existent instance', async ({
+    task,
+    onTestFinished,
+  }) => {
+    const testData = new TestDecisionsDataManager(task.id, onTestFinished);
+
+    const setup = await testData.createDecisionSetup({
+      instanceCount: 0,
+    });
+
+    const caller = await createAuthenticatedCaller(setup.userEmail);
+
+    await expect(
+      caller.decision.deleteInstance({
+        instanceId: '00000000-0000-0000-0000-000000000000',
+      }),
+    ).rejects.toThrow(/not found/i);
+  });
+
+  it('should not allow user with member profile access to delete instance', async ({
+    task,
+    onTestFinished,
+  }) => {
+    const testData = new TestDecisionsDataManager(task.id, onTestFinished);
+
+    // Create instance owned by first user
+    const setup = await testData.createDecisionSetup({
+      instanceCount: 1,
+      grantAccess: true,
+    });
+
+    const { instance, profileId } = setup.instances[0]!;
+
+    // Create a second user with MEMBER access on the decision profile (not admin)
+    const memberUser = await testData.createMemberUser({
+      organization: setup.organization,
+      instanceProfileIds: [profileId], // grants MEMBER role on profile
+    });
+
+    const memberCaller = await createAuthenticatedCaller(memberUser.email);
+
+    // Member should not be able to delete the instance (requires admin profile access)
+    await expect(
+      memberCaller.decision.deleteInstance({
+        instanceId: instance.id,
+      }),
+    ).rejects.toThrow(/access denied|not authorized|unauthorized/i);
+
+    // Verify the instance still exists
+    const existingInstance = await db._query.processInstances.findFirst({
+      where: (table, { eq }) => eq(table.id, instance.id),
+    });
+
+    expect(existingInstance).toBeDefined();
+  });
+
+  it('should allow user with admin profile access to delete instance', async ({
+    task,
+    onTestFinished,
+  }) => {
+    const testData = new TestDecisionsDataManager(task.id, onTestFinished);
+
+    // Create instance owned by first user
+    const setup = await testData.createDecisionSetup({
+      instanceCount: 1,
+      grantAccess: true,
+    });
+
+    const { instance, profileId } = setup.instances[0]!;
+
+    // Create a second user who is not the owner
+    const adminUser = await testData.createMemberUser({
+      organization: setup.organization,
+      // Don't pass instanceProfileIds - we'll grant admin access manually
+    });
+
+    // Grant admin access on the decision profile (not just member)
+    await testData.grantProfileAccess(
+      profileId,
+      adminUser.authUserId,
+      adminUser.email,
+      true, // isAdmin = true
+    );
+
+    const adminCaller = await createAuthenticatedCaller(adminUser.email);
+
+    // User with admin profile access should be able to delete the instance
+    const result = await adminCaller.decision.deleteInstance({
+      instanceId: instance.id,
+    });
+
+    expect(result.success).toBe(true);
+    expect(result.action).toBe('deleted');
+    expect(result.instanceId).toBe(instance.id);
+
+    // Verify the instance was actually deleted
+    const deletedInstance = await db._query.processInstances.findFirst({
+      where: (table, { eq }) => eq(table.id, instance.id),
+    });
+
+    expect(deletedInstance).toBeUndefined();
+  });
+
+  it('should handle already cancelled instance gracefully', async ({
+    task,
+    onTestFinished,
+  }) => {
+    const testData = new TestDecisionsDataManager(task.id, onTestFinished);
+
+    const setup = await testData.createDecisionSetup({
+      instanceCount: 1,
+      grantAccess: true,
+    });
+
+    const { instance } = setup.instances[0]!;
+    const caller = await createAuthenticatedCaller(setup.userEmail);
+
+    // Pre-cancel the instance and add a transition so delete would try to cancel again
+    await db
+      .update(processInstances)
+      .set({ status: ProcessStatus.CANCELLED })
+      .where(eq(processInstances.id, instance.id));
+
+    await db.insert(stateTransitionHistory).values([
+      {
+        processInstanceId: instance.id,
+        fromStateId: 'initial',
+        toStateId: 'final',
+        transitionedAt: new Date(),
+      },
+    ]);
+
+    // Calling delete on an already cancelled instance should still succeed
+    const result = await caller.decision.deleteInstance({
+      instanceId: instance.id,
+    });
+
+    expect(result.success).toBe(true);
+    expect(result.action).toBe('cancelled');
+    expect(result.instanceId).toBe(instance.id);
+
+    // Verify the instance is still cancelled
+    const cancelledInstance = await db._query.processInstances.findFirst({
+      where: (table, { eq }) => eq(table.id, instance.id),
+    });
+
+    expect(cancelledInstance).toBeDefined();
+    expect(cancelledInstance!.status).toBe('cancelled');
+  });
+});
diff --git a/services/api/src/routers/decision/instances/deleteInstance.ts b/services/api/src/routers/decision/instances/deleteInstance.ts
new file mode 100644
index 000000000..15c3a9c2f
--- /dev/null
+++ b/services/api/src/routers/decision/instances/deleteInstance.ts
@@ -0,0 +1,28 @@
+import { deleteInstance as deleteInstanceService } from '@op/common';
+import { z } from 'zod';
+
+import { commonAuthedProcedure, router } from '../../../trpcFactory';
+
+export const deleteInstanceRouter = router({
+  deleteInstance: commonAuthedProcedure()
+    .input(
+      z.object({
+        instanceId: z.string().uuid(),
+      }),
+    )
+    .output(
+      z.object({
+        success: z.boolean(),
+        action: z.enum(['deleted', 'cancelled']),
+        instanceId: z.string(),
+      }),
+    )
+    .mutation(async ({ ctx, input }) => {
+      const { user } = ctx;
+
+      return deleteInstanceService({
+        instanceId: input.instanceId,
+        user,
+      });
+    }),
+});
diff --git a/services/api/src/routers/decision/instances/index.ts b/services/api/src/routers/decision/instances/index.ts
index 492c4ea06..4d6c48127 100644
--- a/services/api/src/routers/decision/instances/index.ts
+++ b/services/api/src/routers/decision/instances/index.ts
@@ -1,6 +1,7 @@
 import { mergeRouters } from '../../../trpcFactory';
 import { createInstanceRouter } from './createInstance';
 import { createInstanceFromTemplateRouter } from './createInstanceFromTemplate';
+import { deleteInstanceRouter } from './deleteInstance';
 import { getCategoriesRouter } from './getCategories';
 import { getDecisionBySlugRouter } from './getDecisionBySlug';
 import { getInstanceRouter, getLegacyInstanceRouter } from './getInstance';
@@ -14,6 +15,7 @@ export const instancesRouter = mergeRouters(
   createInstanceFromTemplateRouter,
   updateInstanceRouter,
   updateDecisionInstanceRouter,
+  deleteInstanceRouter,
   listInstancesRouter,
   getInstanceRouter,
   getLegacyInstanceRouter,