diff --git a/dll/win32/imm32/imm.c b/dll/win32/imm32/imm.c
index 4ac2cdebb6e..e24806c86d4 100644
--- a/dll/win32/imm32/imm.c
+++ b/dll/win32/imm32/imm.c
@@ -928,7 +928,7 @@ PCLIENTIMC WINAPI ImmLockClientImc(HIMC hImc)
         // FIXME: NtUserGetThreadState and enum ThreadStateRoutines are broken.
         pClientImc->unknown = NtUserGetThreadState(13);
 
-        if (!NtUserUpdateInputContext(hImc, 0, pClientImc))
+        if (!NtUserUpdateInputContext(hImc, UIC_CLIENTIMCDATA, (DWORD_PTR)pClientImc))
         {
             Imm32HeapFree(pClientImc);
             return NULL;
diff --git a/sdk/include/reactos/undocuser.h b/sdk/include/reactos/undocuser.h
index ca13a8b0239..3c0ebb3c149 100644
--- a/sdk/include/reactos/undocuser.h
+++ b/sdk/include/reactos/undocuser.h
@@ -296,6 +296,12 @@ MessageBoxTimeoutW(
 
 LPCWSTR WINAPI MB_GetString(IN UINT wBtn);
 
+/* dwType for NtUserUpdateInputContext */
+typedef enum _UPDATE_INPUT_CONTEXT
+{
+    UIC_CLIENTIMCDATA = 0,
+    UIC_IMEWINDOW
+} UPDATE_INPUT_CONTEXT;
 
 //
 // User api hook
diff --git a/win32ss/include/ntuser.h b/win32ss/include/ntuser.h
index 0e6d4c3b4d9..1e6234f84c1 100644
--- a/win32ss/include/ntuser.h
+++ b/win32ss/include/ntuser.h
@@ -3533,12 +3533,12 @@ BOOL
 NTAPI
 NtUserUnregisterUserApiHook(VOID);
 
-DWORD
+BOOL
 NTAPI
 NtUserUpdateInputContext(
     HIMC hIMC,
-    DWORD Unknown1,
-    LPVOID pClientImc);
+    DWORD dwType,
+    DWORD_PTR dwValue);
 
 DWORD
 NTAPI
diff --git a/win32ss/user/ntuser/ntstubs.c b/win32ss/user/ntuser/ntstubs.c
index 1a173714958..95a87900242 100644
--- a/win32ss/user/ntuser/ntstubs.c
+++ b/win32ss/user/ntuser/ntstubs.c
@@ -346,16 +346,61 @@ NtUserSetSysColors(
    return Ret;
 }
 
-DWORD
+BOOL FASTCALL UserUpdateInputContext(PIMC pIMC, DWORD dwType, DWORD_PTR dwValue)
+{
+    PTHREADINFO pti = GetW32ThreadInfo();
+    PTHREADINFO ptiIMC = pIMC->head.pti;
+
+    if (pti->ppi != ptiIMC->ppi) // Different process?
+        return FALSE;
+
+    switch (dwType)
+    {
+        case UIC_CLIENTIMCDATA:
+            if (pIMC->dwClientImcData)
+                return FALSE; // Already set
+
+            pIMC->dwClientImcData = dwValue;
+            break;
+
+        case UIC_IMEWINDOW:
+            if (!ValidateHwndNoErr((HWND)dwValue))
+                return FALSE; // Invalid HWND
+
+            pIMC->hImeWnd = (HWND)dwValue;
+            break;
+
+        default:
+            return FALSE;
+    }
+
+    return TRUE;
+}
+
+BOOL
 APIENTRY
 NtUserUpdateInputContext(
     HIMC hIMC,
-    DWORD Unknown1,
-    LPVOID pClientImc)
+    DWORD dwType,
+    DWORD_PTR dwValue)
 {
-   STUB
+    PIMC pIMC;
+    BOOL ret = FALSE;
 
-   return 0;
+    UserEnterExclusive();
+
+    if (!IS_IMM_MODE())
+        goto Quit;
+
+    pIMC = UserGetObject(gHandleTable, hIMC, TYPE_INPUTCONTEXT);
+    if (!pIMC)
+        goto Quit;
+
+    ret = UserUpdateInputContext(pIMC, dwType, dwValue);
+
+Quit:
+    UserLeave();
+    return ret;
 }
 
 DWORD