diff --git a/packages/common/src/services/decision/deleteInstance.ts b/packages/common/src/services/decision/deleteInstance.ts
new file mode 100644
index 000000000..966a08b5e
--- /dev/null
+++ b/packages/common/src/services/decision/deleteInstance.ts
@@ -0,0 +1,124 @@
+import { db, eq } from '@op/db/client';
+import {
+  ProcessStatus,
+  processInstances,
+  stateTransitionHistory,
+} from '@op/db/schema';
+import { User } from '@op/supabase/lib';
+import { checkPermission, permission } from 'access-zones';
+
+import { CommonError, NotFoundError, UnauthorizedError } from '../../utils';
+import { getOrgAccessUser, getUserSession } from '../access';
+import { assertOrganizationByProfileId } from '../assert';
+
+export interface DeleteInstanceResult {
+  success: boolean;
+  action: 'deleted' | 'archived';
+  instanceId: string;
+}
+
+export const deleteInstance = async ({
+  instanceId,
+  user,
+}: {
+  instanceId: string;
+  user: User;
+}): Promise<DeleteInstanceResult> => {
+  try {
+    const [sessionUser, existingInstance] = await Promise.all([
+      getUserSession({ authUserId: user.id }),
+      db.query.processInstances.findFirst({
+        where: eq(processInstances.id, instanceId),
+      }),
+    ]);
+
+    const { user: dbUser } = sessionUser ?? {};
+
+    if (!dbUser || !dbUser.currentProfileId) {
+      throw new UnauthorizedError('User must have an active profile');
+    }
+
+    if (!existingInstance) {
+      throw new NotFoundError('Process instance not found');
+    }
+
+    // Get organization from process instance owner profile
+    const organization = await assertOrganizationByProfileId(
+      existingInstance.ownerProfileId,
+    );
+
+    // Get user's organization membership and roles
+    const orgUser = await getOrgAccessUser({
+      user,
+      organizationId: organization.id,
+    });
+
+    const hasPermissions = checkPermission(
+      { decisions: permission.ADMIN },
+      orgUser?.roles ?? [],
+    );
+
+    // Only the process owner or admin can delete the instance
+    const isProcessOwner =
+      existingInstance.ownerProfileId === dbUser.currentProfileId;
+
+    if (!hasPermissions && !isProcessOwner) {
+      throw new UnauthorizedError(
+        'Not authorized to delete this process instance',
+      );
+    }
+
+    // Check if any state transitions have occurred
+    const transitions = await db.query.stateTransitionHistory.findFirst({
+      where: eq(stateTransitionHistory.processInstanceId, instanceId),
+    });
+
+    if (transitions) {
+      // Transitions exist - archive instead of delete
+      const [archivedInstance] = await db
+        .update(processInstances)
+        .set({
+          status: ProcessStatus.ARCHIVED,
+          updatedAt: new Date().toISOString(),
+        })
+        .where(eq(processInstances.id, instanceId))
+        .returning();
+
+      if (!archivedInstance) {
+        throw new CommonError('Failed to archive process instance');
+      }
+
+      return {
+        success: true,
+        action: 'archived',
+        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,
+    };
+  } catch (error) {
+    if (
+      error instanceof UnauthorizedError ||
+      error instanceof NotFoundError ||
+      error instanceof CommonError
+    ) {
+      throw error;
+    }
+    console.error('Error deleting process instance:', error);
+    throw new CommonError('Failed to delete process instance');
+  }
+};
diff --git a/packages/common/src/services/decision/index.ts b/packages/common/src/services/decision/index.ts
index 9c78c1fde..e6ee504ad 100644
--- a/packages/common/src/services/decision/index.ts
+++ b/packages/common/src/services/decision/index.ts
@@ -9,6 +9,7 @@ export * from './listProcesses';
 export * from './createInstance';
 export * from './createInstanceFromTemplate';
 export * from './updateInstance';
+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..00657c077
--- /dev/null
+++ b/services/api/src/routers/decision/instances/deleteInstance.test.ts
@@ -0,0 +1,160 @@
+import { db, eq } from '@op/db/client';
+import { 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: eq(processInstances.id, instance.id),
+    });
+
+    expect(deletedInstance).toBeUndefined();
+  });
+
+  it('should archive 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('archived');
+    expect(result.instanceId).toBe(instance.id);
+
+    // Verify the instance was archived, not deleted
+    const archivedInstance = await db.query.processInstances.findFirst({
+      where: eq(processInstances.id, instance.id),
+    });
+
+    expect(archivedInstance).toBeDefined();
+    expect(archivedInstance!.status).toBe('archived');
+  });
+
+  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();
+  });
+
+  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 non-owner 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 (just a member)
+    const memberUser = await testData.createMemberUser({
+      organization: setup.organization,
+      instanceProfileIds: [profileId],
+    });
+
+    const memberCaller = await createAuthenticatedCaller(memberUser.email);
+
+    // Member should not be able to delete the instance
+    await expect(
+      memberCaller.decision.deleteInstance({
+        instanceId: instance.id,
+      }),
+    ).rejects.toThrow(/not authorized/i);
+
+    // Verify the instance still exists
+    const existingInstance = await db.query.processInstances.findFirst({
+      where: eq(processInstances.id, instance.id),
+    });
+
+    expect(existingInstance).toBeDefined();
+  });
+});
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..b8ec2ad59
--- /dev/null
+++ b/services/api/src/routers/decision/instances/deleteInstance.ts
@@ -0,0 +1,87 @@
+import {
+  NotFoundError,
+  UnauthorizedError,
+  deleteInstance as deleteInstanceService,
+} from '@op/common';
+import { TRPCError } from '@trpc/server';
+import type { OpenApiMeta } from 'trpc-to-openapi';
+import { z } from 'zod';
+
+import withAnalytics from '../../../middlewares/withAnalytics';
+import withAuthenticated from '../../../middlewares/withAuthenticated';
+import withRateLimited from '../../../middlewares/withRateLimited';
+import { loggedProcedure, router } from '../../../trpcFactory';
+
+const meta: OpenApiMeta = {
+  openapi: {
+    enabled: true,
+    method: 'DELETE',
+    path: '/decision/instance/{instanceId}',
+    protect: true,
+    tags: ['decision'],
+    summary: 'Delete or archive process instance',
+  },
+};
+
+export const deleteInstanceRouter = router({
+  deleteInstance: loggedProcedure
+    .use(withRateLimited({ windowSize: 10, maxRequests: 5 }))
+    .use(withAuthenticated)
+    .use(withAnalytics)
+    .meta(meta)
+    .input(
+      z.object({
+        instanceId: z.string().uuid(),
+      }),
+    )
+    .output(
+      z.object({
+        success: z.boolean(),
+        action: z.enum(['deleted', 'archived']),
+        instanceId: z.string(),
+      }),
+    )
+    .mutation(async ({ ctx, input }) => {
+      const { user, logger } = ctx;
+
+      try {
+        const result = await deleteInstanceService({
+          instanceId: input.instanceId,
+          user,
+        });
+
+        logger.info('Process instance deleted/archived', {
+          userId: user.id,
+          instanceId: input.instanceId,
+          action: result.action,
+        });
+
+        return result;
+      } catch (error: unknown) {
+        logger.error('Failed to delete process instance', {
+          userId: user.id,
+          instanceId: input.instanceId,
+          error,
+        });
+
+        if (error instanceof UnauthorizedError) {
+          throw new TRPCError({
+            message: error.message,
+            code: 'UNAUTHORIZED',
+          });
+        }
+
+        if (error instanceof NotFoundError) {
+          throw new TRPCError({
+            message: error.message,
+            code: 'NOT_FOUND',
+          });
+        }
+
+        throw new TRPCError({
+          message: 'Failed to delete process instance',
+          code: 'INTERNAL_SERVER_ERROR',
+        });
+      }
+    }),
+});
diff --git a/services/api/src/routers/decision/instances/index.ts b/services/api/src/routers/decision/instances/index.ts
index 513c448e4..864fa3676 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 } from './getInstance';
@@ -12,6 +13,7 @@ export const instancesRouter = mergeRouters(
   createInstanceRouter,
   createInstanceFromTemplateRouter,
   updateInstanceRouter,
+  deleteInstanceRouter,
   listInstancesRouter,
   getInstanceRouter,
   getCategoriesRouter,