diff --git a/packages/server/src/utils/inviteService.test.ts b/packages/server/src/utils/inviteService.test.ts
index 56f39865..4cd0e501 100644
--- a/packages/server/src/utils/inviteService.test.ts
+++ b/packages/server/src/utils/inviteService.test.ts
@@ -23,6 +23,7 @@ vi.mock('../db/index.js', () => ({
 
 import { inviteStatus, generateInviteToken, createInvite, getInviteByToken, listInvites, listRedemptions, InviteValidationError } from './inviteService.js';
 import { patchInvite, revokeInvite, InviteStateConflictError, InviteNotFoundError } from './inviteService.js';
+import { reinstateInvite } from './inviteService.js';
 import { eq } from 'drizzle-orm';
 
 function applyMigrations(db: Database.Database): void {
@@ -374,3 +375,74 @@ describe('revokeInvite', () => {
     expect(() => revokeInvite('nonexistent')).toThrow(InviteNotFoundError);
   });
 });
+
+describe('reinstateInvite', () => {
+  beforeEach(() => {
+    sqlite = new Database(':memory:');
+    sqlite.pragma('foreign_keys = ON');
+    applyMigrations(sqlite);
+    testDb = drizzle(sqlite, { schema });
+  });
+
+  it('Path A — was revoked: rotates token, clears revokedAt, applies bumps', () => {
+    const adminId = seedAdmin();
+    const inv = createInvite({ name: 'a', maxUses: 5, expiresAt: null }, adminId);
+    const originalToken = inv.token;
+    revokeInvite(inv.id);
+
+    const result = reinstateInvite(inv.id, { maxUses: 10 });
+    expect(result.tokenRotated).toBe(true);
+    expect(result.invite.token).not.toBe(originalToken);
+    expect(result.invite.token).toMatch(/^[A-Za-z0-9_-]{22}$/);
+    expect(result.invite.revokedAt).toBeNull();
+    expect(result.invite.maxUses).toBe(10);
+    expect(result.invite.status).toBe('active');
+  });
+
+  it('Path B — exhausted: keeps same token, applies maxUses bump', () => {
+    const adminId = seedAdmin();
+    const inv = createInvite({ name: 'a', maxUses: 1, expiresAt: null }, adminId);
+    // Exhaust it
+    testDb.update(schema.inviteLinks).set({ usedCount: 1 }).where(eq(schema.inviteLinks.id, inv.id)).run();
+
+    const result = reinstateInvite(inv.id, { maxUses: 5 });
+    expect(result.tokenRotated).toBe(false);
+    expect(result.invite.token).toBe(inv.token);
+    expect(result.invite.maxUses).toBe(5);
+    expect(result.invite.status).toBe('active');
+  });
+
+  it('Path B — expired: keeps same token, bumps expiresAt', () => {
+    const adminId = seedAdmin();
+    const inv = createInvite({ name: 'a', maxUses: null, expiresAt: Date.now() + 100_000 }, adminId);
+    // Move to past to expire
+    testDb.update(schema.inviteLinks).set({ expiresAt: Date.now() - 1000 }).where(eq(schema.inviteLinks.id, inv.id)).run();
+
+    const future = Date.now() + 86_400_000;
+    const result = reinstateInvite(inv.id, { expiresAt: future });
+    expect(result.tokenRotated).toBe(false);
+    expect(result.invite.token).toBe(inv.token);
+    expect(result.invite.expiresAt).toBe(future);
+    expect(result.invite.status).toBe('active');
+  });
+
+  it('Path C — already-active: throws InviteStateConflictError', () => {
+    const adminId = seedAdmin();
+    const inv = createInvite({ name: 'a', maxUses: null, expiresAt: null }, adminId);
+    expect(() => reinstateInvite(inv.id, {})).toThrow(InviteStateConflictError);
+  });
+
+  it('throws InviteValidationError when result is still non-active (caller did not bump enough)', () => {
+    const adminId = seedAdmin();
+    const inv = createInvite({ name: 'a', maxUses: 1, expiresAt: null }, adminId);
+    // Exhaust it
+    testDb.update(schema.inviteLinks).set({ usedCount: 1 }).where(eq(schema.inviteLinks.id, inv.id)).run();
+
+    // Caller did not bump maxUses → still exhausted → must throw
+    expect(() => reinstateInvite(inv.id, {})).toThrow(InviteValidationError);
+
+    // Verify the txn rolled back: row still has maxUses=1 (no partial update)
+    const row = testDb.select().from(schema.inviteLinks).where(eq(schema.inviteLinks.id, inv.id)).get();
+    expect(row?.maxUses).toBe(1);
+  });
+});
diff --git a/packages/server/src/utils/inviteService.ts b/packages/server/src/utils/inviteService.ts
index fe3190e8..d318227f 100644
--- a/packages/server/src/utils/inviteService.ts
+++ b/packages/server/src/utils/inviteService.ts
@@ -3,7 +3,14 @@ import { eq, desc } from 'drizzle-orm';
 import { getDb, schema } from '../db/index.js';
 import { generateSnowflake } from './snowflake.js';
 import { config } from '../config.js';
-import type { InviteLinkSummary, CreateInviteRequest, InviteRedemption, UpdateInviteRequest } from '@backspace/shared';
+import type {
+  InviteLinkSummary,
+  CreateInviteRequest,
+  InviteRedemption,
+  UpdateInviteRequest,
+  ReinstateInviteRequest,
+  ReinstateInviteResponse,
+} from '@backspace/shared';
 
 /**
  * Derived status of an invite link. Mirrors the `InviteStatus` union exported
@@ -381,3 +388,76 @@ export function revokeInvite(id: string): InviteLinkSummary {
     return rowToSummary(updated, resolveCreatorUsername(updated.createdBy, tx));
   });
 }
+
+/**
+ * Reinstate a non-active invite back to `active`. Three branches per spec §3.1:
+ *
+ *   - **Path A (revoked)**: rotates the token (security boundary — old shared
+ *     links must stop working) and clears `revokedAt`. Caller may also bump
+ *     `maxUses` / `expiresAt` in the same call.
+ *   - **Path B (expired/exhausted)**: preserves the token. Caller MUST supply
+ *     bumps that push the row back into derived `active` state, otherwise the
+ *     txn rolls back with `InviteValidationError` (we never leave an invite
+ *     half-reinstated, e.g. exhausted-and-still-exhausted with no token rotation
+ *     and no state change).
+ *   - **Path C (already active)**: rejected with `InviteStateConflictError`
+ *     (mapped to 409). Reinstating an active invite is meaningless and would
+ *     surprise an admin who clicked the wrong row.
+ *
+ * Wrapped in a SQLite transaction with an in-txn re-read so the post-update
+ * status check sees the row as the next reader would. If the post-state isn't
+ * `active`, the throw aborts the txn and the row reverts.
+ */
+export function reinstateInvite(id: string, req: ReinstateInviteRequest): ReinstateInviteResponse {
+  const db = getDb();
+  return db.transaction((tx) => {
+    const row = tx.select().from(schema.inviteLinks).where(eq(schema.inviteLinks.id, id)).get();
+    if (!row) throw new InviteNotFoundError();
+
+    const currentStatus = inviteStatus(row);
+    if (currentStatus === 'active') {
+      throw new InviteStateConflictError('Invite is already active');
+    }
+
+    const updates: Partial<typeof schema.inviteLinks.$inferInsert> = {};
+    let tokenRotated = false;
+
+    if (currentStatus === 'revoked') {
+      updates.revokedAt = null;
+      updates.token = generateInviteToken();
+      tokenRotated = true;
+    }
+
+    if (req.maxUses !== undefined) {
+      const v = validateMaxUses(req.maxUses);
+      if (v !== null && v < row.usedCount) {
+        throw new InviteValidationError(
+          `maxUses (${v}) cannot be less than current usedCount (${row.usedCount})`,
+        );
+      }
+      updates.maxUses = v;
+    }
+    if (req.expiresAt !== undefined) {
+      updates.expiresAt = validateExpiresAt(req.expiresAt, true);
+    }
+
+    if (Object.keys(updates).length > 0) {
+      tx.update(schema.inviteLinks).set(updates).where(eq(schema.inviteLinks.id, id)).run();
+    }
+
+    const updated = tx.select().from(schema.inviteLinks).where(eq(schema.inviteLinks.id, id)).get();
+    if (!updated) throw new Error('Failed to read updated invite');
+
+    if (inviteStatus(updated) !== 'active') {
+      // Caller did not bump enough — abort the txn so nothing is half-applied
+      throw new InviteValidationError(
+        'Reinstate would leave invite in non-active state. Bump maxUses and/or expiresAt.',
+      );
+    }
+
+    return {
+      invite: rowToSummary(updated, resolveCreatorUsername(updated.createdBy, tx)),
+      tokenRotated,
+    };
+  });
+}