diff --git a/app/server/modules/auth/__tests__/auth.cleanup-user-organizations.test.ts b/app/server/modules/auth/__tests__/auth.cleanup-user-organizations.test.ts
new file mode 100644
index 00000000..8882536b
--- /dev/null
+++ b/app/server/modules/auth/__tests__/auth.cleanup-user-organizations.test.ts
@@ -0,0 +1,123 @@
+import { beforeEach, describe, expect, test } from "bun:test";
+import { db } from "~/server/db/db";
+import { member, organization, sessionsTable, usersTable } from "~/server/db/schema";
+import { authService } from "../auth.service";
+
+function randomId() {
+	return Bun.randomUUIDv7();
+}
+
+function randomSlug(prefix: string) {
+	return `${prefix}-${Math.random().toString(36).slice(2, 8)}`;
+}
+
+async function createUser(email: string) {
+	const id = randomId();
+
+	await db.insert(usersTable).values({
+		id,
+		email,
+		name: email.split("@")[0],
+		username: randomSlug("user"),
+	});
+
+	return id;
+}
+
+async function createOrganization(name: string) {
+	const id = randomId();
+
+	await db.insert(organization).values({
+		id,
+		name,
+		slug: randomSlug("org"),
+		createdAt: new Date(),
+	});
+
+	return id;
+}
+
+async function createMembership({
+	userId,
+	organizationId,
+	role,
+}: {
+	userId: string;
+	organizationId: string;
+	role: "owner" | "admin" | "member";
+}) {
+	await db.insert(member).values({
+		id: randomId(),
+		userId,
+		organizationId,
+		role,
+		createdAt: new Date(),
+	});
+}
+
+async function createSession({
+	userId,
+	activeOrganizationId,
+}: {
+	userId: string;
+	activeOrganizationId: string | null;
+}) {
+	const id = randomId();
+
+	await db.insert(sessionsTable).values({
+		id,
+		userId,
+		token: randomSlug("token"),
+		expiresAt: new Date(Date.now() + 60_000),
+		activeOrganizationId,
+	});
+
+	return id;
+}
+
+describe("authService.cleanupUserOrganizations", () => {
+	beforeEach(async () => {
+		await db.delete(member);
+		await db.delete(sessionsTable);
+		await db.delete(organization);
+		await db.delete(usersTable);
+	});
+
+	test("reassigns active organization for members whose active org gets deleted", async () => {
+		const adminUserId = await createUser(`${randomSlug("admin")}@example.com`);
+		const deletedUserId = await createUser(`${randomSlug("deleted")}@example.com`);
+
+		const adminWorkspaceId = await createOrganization("Admin Workspace");
+		const deletedWorkspaceId = await createOrganization("Deleted Workspace");
+
+		await createMembership({ userId: adminUserId, organizationId: adminWorkspaceId, role: "owner" });
+		await createMembership({ userId: deletedUserId, organizationId: deletedWorkspaceId, role: "owner" });
+		await createMembership({ userId: adminUserId, organizationId: deletedWorkspaceId, role: "member" });
+
+		const adminSessionId = await createSession({
+			userId: adminUserId,
+			activeOrganizationId: deletedWorkspaceId,
+		});
+
+		await authService.cleanupUserOrganizations(deletedUserId);
+
+		const remainingSession = await db.query.sessionsTable.findFirst({
+			where: { id: adminSessionId },
+			columns: { activeOrganizationId: true },
+		});
+		const deletedWorkspace = await db.query.organization.findFirst({
+			where: { id: deletedWorkspaceId },
+			columns: { id: true },
+		});
+
+		expect(deletedWorkspace).toBeUndefined();
+		expect(remainingSession?.activeOrganizationId).toBe(adminWorkspaceId);
+
+		const deletedMembership = await db.query.member.findFirst({
+			where: { AND: [{ organizationId: deletedWorkspaceId }, { userId: adminUserId }] },
+			columns: { id: true },
+		});
+
+		expect(deletedMembership).toBeUndefined();
+	});
+});
diff --git a/app/server/modules/auth/auth.service.ts b/app/server/modules/auth/auth.service.ts
index 215387b8..8c3582df 100644
--- a/app/server/modules/auth/auth.service.ts
+++ b/app/server/modules/auth/auth.service.ts
@@ -3,6 +3,7 @@ import {
 	usersTable,
 	member,
 	organization,
+	sessionsTable,
 	volumesTable,
 	repositoriesTable,
 	backupSchedulesTable,
@@ -101,9 +102,43 @@ export class AuthService {
 		const impact = await this.getUserDeletionImpact(userId);
 		const orgIds = impact.organizations.map((o) => o.id);
 
-		if (orgIds.length > 0) {
-			await db.delete(organization).where(inArray(organization.id, orgIds));
+		if (orgIds.length === 0) {
+			return;
 		}
+
+		await db.transaction(async (tx) => {
+			const membersInDeletedOrgs = await tx
+				.select({ userId: member.userId })
+				.from(member)
+				.where(and(inArray(member.organizationId, orgIds), ne(member.userId, userId)));
+
+			const affectedUserIds = [...new Set(membersInDeletedOrgs.map((r) => r.userId))];
+
+			if (affectedUserIds.length > 0) {
+				const memberships = await tx
+					.select({ userId: member.userId, organizationId: member.organizationId })
+					.from(member)
+					.where(inArray(member.userId, affectedUserIds));
+
+				const orgIdSet = new Set(orgIds);
+				const fallbackOrgByUser = new Map<string, string | null>(affectedUserIds.map((id) => [id, null]));
+
+				for (const { userId, organizationId } of memberships) {
+					if (!orgIdSet.has(organizationId) && fallbackOrgByUser.get(userId) === null) {
+						fallbackOrgByUser.set(userId, organizationId);
+					}
+				}
+
+				for (const [affectedUserId, fallbackOrgId] of fallbackOrgByUser) {
+					await tx
+						.update(sessionsTable)
+						.set({ activeOrganizationId: fallbackOrgId })
+						.where(and(eq(sessionsTable.userId, affectedUserId), inArray(sessionsTable.activeOrganizationId, orgIds)));
+				}
+			}
+
+			await tx.delete(organization).where(inArray(organization.id, orgIds));
+		});
 	}
 
 	/**