diff --git a/frontend/src/app/departments/[id]/page.tsx b/frontend/src/app/departments/[id]/page.tsx
index 47511674..e041d42f 100644
--- a/frontend/src/app/departments/[id]/page.tsx
+++ b/frontend/src/app/departments/[id]/page.tsx
@@ -406,6 +406,7 @@ function AddMemberModal({
   isOpen,
   items,
   loading,
+  memberError,
   mode,
   optionsLoading,
   onClose,
@@ -423,6 +424,7 @@ function AddMemberModal({
   isOpen: boolean;
   items: User[];
   loading: boolean;
+  memberError: string | null;
   mode: DepartmentMemberModalMode;
   optionsLoading: boolean;
   onClose: () => void;
@@ -592,6 +594,12 @@ function AddMemberModal({
           </div>
         ) : null}
 
+        {!directoryError && memberError ? (
+          <div className="app-feedback-danger rounded-xl p-3 text-sm">
+            <p>{memberError}</p>
+          </div>
+        ) : null}
+
         {canAssignRoles && !directoryError ? (
           <div className="space-y-3">
             <SearchableSelectSingle
@@ -978,6 +986,7 @@ export default function DepartmentDetailPage() {
           h.assignableEmployees,
         )}
         loading={h.pendingKey === "member"}
+        memberError={h.memberModalError}
         mode={h.memberModalMode}
         optionsLoading={h.employeesDirectoryLoading}
         onClose={h.closeAddMember}
diff --git a/frontend/src/components/departments/member-modal-state.js b/frontend/src/components/departments/member-modal-state.js
index 30b7e272..ec1c6de3 100644
--- a/frontend/src/components/departments/member-modal-state.js
+++ b/frontend/src/components/departments/member-modal-state.js
@@ -55,3 +55,31 @@ export function isDepartmentMemberModalSubmitDisabled({
   if (mode === "assignRole" && !selectedRoleId) return true;
   return false;
 }
+
+export function getDepartmentMemberModalError(message, mode) {
+  const normalized = String(message || "").trim();
+  if (!normalized) return null;
+
+  const anotherDepartmentMatch = normalized.match(
+    /Employee already belongs to another active department:\s*(.+?)\.?$/i,
+  );
+  if (anotherDepartmentMatch) {
+    const departmentName = anotherDepartmentMatch[1]?.trim();
+    if (mode === "assignRole") {
+      return null;
+    }
+    return departmentName
+      ? `Сотрудник уже состоит в другом активном отделе: ${departmentName}.`
+      : "Сотрудник уже состоит в другом активном отделе.";
+  }
+
+  if (/Employee is inactive\.?$/i.test(normalized)) {
+    return "Можно выбрать только активного сотрудника.";
+  }
+
+  if (/Employee not found\.?$/i.test(normalized)) {
+    return "Сотрудник не найден.";
+  }
+
+  return null;
+}
diff --git a/frontend/src/components/departments/member-modal-state.test.js b/frontend/src/components/departments/member-modal-state.test.js
index 62eb69c5..6b313a65 100644
--- a/frontend/src/components/departments/member-modal-state.test.js
+++ b/frontend/src/components/departments/member-modal-state.test.js
@@ -2,6 +2,7 @@ import test from "node:test";
 import assert from "node:assert/strict";
 
 import {
+  getDepartmentMemberModalError,
   getDepartmentMemberModalEmployeeLabel,
   getDepartmentMemberModalHelperText,
   getDepartmentMemberModalItems,
@@ -92,3 +93,24 @@ test("member modal helper texts reflect mode and loading state", () => {
     /любому активному сотруднику/i,
   );
 });
+
+test("member modal maps backend department validation to localized inline error", () => {
+  assert.equal(
+    getDepartmentMemberModalError(
+      "Employee already belongs to another active department: Бухгалтерия.",
+      "add",
+    ),
+    "Сотрудник уже состоит в другом активном отделе: Бухгалтерия.",
+  );
+  assert.equal(
+    getDepartmentMemberModalError("Employee is inactive.", "add"),
+    "Можно выбрать только активного сотрудника.",
+  );
+  assert.equal(
+    getDepartmentMemberModalError(
+      "Employee already belongs to another active department: Бухгалтерия.",
+      "assignRole",
+    ),
+    null,
+  );
+});
diff --git a/frontend/src/hooks/useDepartmentPage.ts b/frontend/src/hooks/useDepartmentPage.ts
index 48953861..e1ab6b64 100644
--- a/frontend/src/hooks/useDepartmentPage.ts
+++ b/frontend/src/hooks/useDepartmentPage.ts
@@ -3,6 +3,7 @@
 import { useCallback, useEffect, useMemo, useState } from "react";
 import { toast } from "sonner";
 
+import { getDepartmentMemberModalError } from "@/components/departments/member-modal-state";
 import { useUser } from "@/contexts/UserContext";
 import { apiClient } from "@/lib/api";
 import { loadAllPages } from "@/lib/shared";
@@ -48,6 +49,7 @@ export type DepartmentPageController = {
   employeesDirectoryLoading: boolean;
   loading: boolean;
   memberModalMode: DepartmentMemberModalMode;
+  memberModalError: string | null;
   members: DepartmentMemberLink[];
   pendingKey: SavingKey | null;
   roleDraft: DepartmentRoleDraft;
@@ -148,6 +150,7 @@ export function useDepartmentPage(departmentId: number): DepartmentPageControlle
 
   const [addMemberOpen, setAddMemberOpen] = useState(false);
   const [memberModalMode, setMemberModalMode] = useState<DepartmentMemberModalMode>("add");
+  const [memberModalError, setMemberModalError] = useState<string | null>(null);
   const [selectedMemberId, setSelectedMemberId] = useState<number | null>(null);
   const [selectedRoleId, setSelectedRoleId] = useState<number | null>(null);
 
@@ -345,6 +348,7 @@ export function useDepartmentPage(departmentId: number): DepartmentPageControlle
 
   const openAddMember = useCallback(() => {
     setMemberModalMode("add");
+    setMemberModalError(null);
     setSelectedMemberId(null);
     setSelectedRoleId(null);
     setAddMemberOpen(true);
@@ -357,6 +361,7 @@ export function useDepartmentPage(departmentId: number): DepartmentPageControlle
 
   const openAssignRoleModal = useCallback(() => {
     setMemberModalMode("assignRole");
+    setMemberModalError(null);
     setSelectedMemberId(null);
     setSelectedRoleId(null);
     setAddMemberOpen(true);
@@ -368,12 +373,23 @@ export function useDepartmentPage(departmentId: number): DepartmentPageControlle
   }, [allEmployees.length, loadEmployeesDirectory]);
 
   const closeAddMember = useCallback(() => {
+    setMemberModalError(null);
     setSelectedMemberId(null);
     setSelectedRoleId(null);
     setMemberModalMode("add");
     setAddMemberOpen(false);
   }, []);
 
+  const selectMember = useCallback((id: number | null) => {
+    setMemberModalError(null);
+    setSelectedMemberId(id);
+  }, []);
+
+  const selectRole = useCallback((id: number | null) => {
+    setMemberModalError(null);
+    setSelectedRoleId(id);
+  }, []);
+
   const submitAddMember = useCallback(async () => {
     if (!department || !selectedMemberId) {
       toast.error("Выберите сотрудника");
@@ -403,6 +419,7 @@ export function useDepartmentPage(departmentId: number): DepartmentPageControlle
       }
       await loadCore();
       setAddMemberOpen(false);
+      setMemberModalError(null);
       setSelectedMemberId(null);
       setSelectedRoleId(null);
       setMemberModalMode("add");
@@ -412,14 +429,21 @@ export function useDepartmentPage(departmentId: number): DepartmentPageControlle
           : "Роль назначена сотруднику",
       );
     } catch (submitError) {
-      toast.error(
-        getErrorMessage(
-          submitError,
-          memberModalMode === "add"
-            ? "Не удалось добавить сотрудника"
-            : "Не удалось назначить роль сотруднику",
-        ),
+      const message = getErrorMessage(
+        submitError,
+        memberModalMode === "add"
+          ? "Не удалось добавить сотрудника"
+          : "Не удалось назначить роль сотруднику",
       );
+      const inlineMessage = getDepartmentMemberModalError(
+        message,
+        memberModalMode,
+      );
+      if (inlineMessage) {
+        setMemberModalError(inlineMessage);
+      } else {
+        toast.error(message);
+      }
     } finally {
       setPendingKey(null);
     }
@@ -624,6 +648,7 @@ export function useDepartmentPage(departmentId: number): DepartmentPageControlle
     headCandidates,
     loading,
     memberModalMode,
+    memberModalError,
     members,
     pendingKey,
     roleDraft,
@@ -650,8 +675,8 @@ export function useDepartmentPage(departmentId: number): DepartmentPageControlle
     setRoleDraft,
     setMemberModalMode,
     setSelectedHeadId,
-    setSelectedMemberId,
-    setSelectedRoleId,
+    setSelectedMemberId: selectMember,
+    setSelectedRoleId: selectRole,
     submitAddMember,
     submitDeleteDepartment,
     submitHeadChange,