diff --git a/core/libs/prx/libSceNpManager/Export.cpp b/core/libs/prx/libSceNpManager/Export.cpp
index 511383f4c83..f7bf0f725f2 100644
--- a/core/libs/prx/libSceNpManager/Export.cpp
+++ b/core/libs/prx/libSceNpManager/Export.cpp
@@ -210,9 +210,9 @@ int APS5_VABI sceNpUnregisterPremiumEventCallback(void) {
     return 0;
 }
 
-int APS5_VABI sceNpGetUserIdByAccountId() {
-    NotImplemented_nid_no_patch(__func__);
-    return 0;
+int APS5_VABI sceNpGetUserIdByAccountId(uint64_t accountId, int* userId) {
+    if (accountId == 0 || !userId) return SCE_NP_ERROR_INVALID_ARGUMENT;
+    return SCE_NP_ERROR_SIGNED_OUT;
 }
 
 }
diff --git a/core/libs/tests/GuestNpManager.cpp b/core/libs/tests/GuestNpManager.cpp
index 106a1e05b8d..dfe11d6564f 100644
--- a/core/libs/tests/GuestNpManager.cpp
+++ b/core/libs/tests/GuestNpManager.cpp
@@ -6,6 +6,7 @@
 #include <cstring>
 
 extern "C" {
+int APS5_VABI sceNpGetUserIdByAccountId(uint64_t accountId, int* userId);
 int APS5_VABI sceNpGetNpId(int user_id, NpId* np_id);
 }
 
@@ -24,6 +25,11 @@ void Require(bool condition, const char* message) {
 }
 
 int main() {
+    int userId = 0x7a7a;
+    Require(sceNpGetUserIdByAccountId(0x1234, &userId) == SignedOut, "sceNpGetUserIdByAccountId must report the account as signed out");
+    Require(userId == 0x7a7a, "sceNpGetUserIdByAccountId must leave the user id untouched");
+    Require(sceNpGetUserIdByAccountId(0, &userId) == InvalidArgument, "sceNpGetUserIdByAccountId must reject account id 0");
+    Require(sceNpGetUserIdByAccountId(0x1234, nullptr) == InvalidArgument, "sceNpGetUserIdByAccountId must reject a null user id");
     NpId npId{};
     std::memset(&npId, 0x5a, sizeof(npId));
     NpId untouched{};
diff --git a/docs/dev/TechnicalDebt.md b/docs/dev/TechnicalDebt.md
index 626b7958a63..d2875ca6efa 100644
--- a/docs/dev/TechnicalDebt.md
+++ b/docs/dev/TechnicalDebt.md
@@ -86,6 +86,7 @@ Throughout the project, every function at every stage either **does exactly what
 - [sceCoredumpWriteUserString](../../core/libs/prx/libkernel/System/src/Coredump.cpp) and [sceSaveDataConvert, sceSaveDataGetConvertProgress](../../core/libs/prx/libSceSaveData.native/Export.cpp) - imported by PPSA16064, not called during its start-up and added without an implementation; unknown signatures
 - [sce::Json::Parser::parse](../../core/libs/prx/libSceJson2/Export.cpp) (libSceJson2) - a number that overflows a double (e.g. `1e309`) throws: whether the console rejects the document or stores an infinite real is unknown
 - [sceUserServiceGetNpAccountId](../../core/libs/prx/libSceUserService/Export.cpp) (libSceUserService) - the account id for a user with no linked PSN account is assumed to be 0 with success; libSceNpManager's `sceNpGetAccountIdA` answers the same question with `SCE_NP_ERROR_SIGNED_OUT`
+- [sceNpGetUserIdByAccountId](../../core/libs/prx/libSceNpManager/Export.cpp) (libSceNpManager) - the signature `(accountId, userId*)` and the errors for a zero account id or a null pointer are taken from [shadPS4](https://github.com/shadps4-emu/shadPS4/blob/945dbc3cc3eee80ac3e053b438502ed936fa6bb2/src/core/libraries/np/np_manager.cpp); not confirmed on a PS5 title (PPSA14632 imports it). No account is ever signed in here, so every valid lookup returns `SCE_NP_ERROR_SIGNED_OUT`
 - [sceUserServiceGetUserColor](../../core/libs/prx/libSceUserService/Export.cpp) (libSceUserService) - the default profile colour is assumed to be blue (0)
 - [kP2L8t3j-aM](../../core/libs/prx/libSceVideoOut/src/Output.cpp) (libSceVideoOut) - unknown name, signature
 - [LibwuIonIBw](../../core/libs/prx/libSceVideoOut/src/Output.cpp) (libSceVideoOut) - unknown name, signature