diff --git a/core/smn_player.cpp b/core/smn_player.cpp
index 95490bc1..6a052f13 100644
--- a/core/smn_player.cpp
+++ b/core/smn_player.cpp
@@ -37,6 +37,15 @@
 #include <inetchannel.h>
 #include <iclient.h>
 
+#ifndef PRIu64
+#ifdef _WIN32
+#define PRIu64 "I64u"
+#else
+#define PRIu64 "llu"
+#endif
+#endif
+
+
 static cell_t sm_GetMaxHumanPlayers(IPluginContext *pCtx, const cell_t *params)
 {
 	int maxHumans = -1;
@@ -380,6 +389,111 @@ static cell_t RunAdminCacheChecks(IPluginContext *pContext, const cell_t *params
 	return (id != pPlayer->GetAdminId()) ? 1 : 0;
 }
 
+// Must match clients.inc
+enum AuthIdType
+{
+	AuthType_Engine = 0,
+	AuthType_Steam2,
+	AuthType_Steam3,
+	AuthType_SteamId64,
+};
+
+static cell_t GetClientAuthId(IPluginContext *pContext, const cell_t *params)
+{
+	CPlayer *pPlayer = g_Players.GetPlayerByIndex(params[1]);
+	if (!pPlayer)
+	{
+		return pContext->ThrowNativeError("Client index %d is invalid", params[1]);
+	}
+	else if (!pPlayer->IsConnected())
+	{
+		return pContext->ThrowNativeError("Client %d is not connected", params[1]);
+	}
+	
+	switch (params[2])
+	{
+	case AuthType_Engine:
+		{
+			const char *authstr = pPlayer->GetAuthString(params[5]);
+			if (!authstr || authstr[0] == '\0')
+			{
+				return 0;
+			}
+			pContext->StringToLocal(params[3], params[4], authstr);
+		}
+		break;
+	case AuthType_Steam2:
+	case AuthType_Steam3:
+		{
+			if (pPlayer->IsFakeClient())
+			{
+				pContext->StringToLocal(params[3], params[4], "BOT");
+				return 1;
+			}
+			
+			static char authstr[64];
+			unsigned int acctId = pPlayer->GetSteamAccountID(params[5]);
+			if (acctId == 0)
+			{
+				if (g_HL2.IsLANServer())
+				{
+					pContext->StringToLocal(params[3], params[4], "STEAM_ID_LAN");
+					return 1;
+				}
+				else if (!params[5])
+				{
+					pContext->StringToLocal(params[3], params[4], "STEAM_ID_PENDING");
+					return 1;
+				}
+				
+				return 0;
+			}
+			
+			if (params[2] == AuthType_Steam2)
+			{
+#if SOURCE_ENGINE <= SE_LEFT4DEAD
+				unsigned int universe = 0;
+#else
+				unsigned int universe = 1;
+#endif
+				_snprintf(authstr, sizeof(authstr), "STEAM_%u:%u:%u", universe, acctId & 1, acctId >> 1);
+				pContext->StringToLocal(params[3], params[4], authstr);
+				break;
+			}
+			else
+			{
+				_snprintf(authstr, sizeof(authstr), "[U:1:%u]", acctId);
+				pContext->StringToLocal(params[3], params[4], authstr);
+			}
+		}
+		break;
+	case AuthType_SteamId64:
+		{
+			if (pPlayer->IsFakeClient() || g_HL2.IsLANServer())
+			{
+				return 0;
+			}
+			unsigned int acctId = pPlayer->GetSteamAccountID(params[5]);
+			if (acctId == 0)
+			{
+				return 0;
+			}
+			
+			uint64_t steamId = acctId;
+			steamId |= ((uint64_t)1<<32); // Instance (1/Desktop)
+			steamId |= ((uint64_t)1<<52); // Type (1/Individual)
+			steamId |= ((uint64_t)1<<56); // Universe (1/Public)
+			
+			static char authstr[64];
+			snprintf(authstr, sizeof(authstr), "%" PRIu64, steamId);
+			pContext->StringToLocal(params[3], params[4], authstr);
+		}
+		break;
+	}
+	
+	return 1;
+}
+
 REGISTER_NATIVES(playernatives)
 {
 	{"GetMaxHumanPlayers",		sm_GetMaxHumanPlayers},
@@ -393,6 +507,7 @@ REGISTER_NATIVES(playernatives)
 	{"GetClientAvgData",		GetAvgData},
 	{"GetClientAvgPackets",		GetAvgPackets},
 	{"RunAdminCacheChecks",		RunAdminCacheChecks},
+	{"GetClientAuthId",			GetClientAuthId},
 	{NULL,						NULL}
 };
 
diff --git a/plugins/include/clients.inc b/plugins/include/clients.inc
index 3627e971..dc532fed 100644
--- a/plugins/include/clients.inc
+++ b/plugins/include/clients.inc
@@ -45,6 +45,25 @@ enum NetFlow
 	NetFlow_Both,			/**< Both values added together */
 };
 
+/**
+ * Auth string types.
+ *
+ * Note that for the Steam2 and Steam3 types, the following ids are
+ * also valid values:
+ * "STEAM_ID_PENDING" - Authentication is pending.
+ * "STEAM_ID_LAN" - Authentication is disabled because of being on a LAN server.
+ * "BOT" - The client is a bot.
+ */
+enum AuthIdType
+{
+	AuthId_Engine = 0, /**< The game-specific auth string as returned from the engine */
+
+	// The following are only available on games that support Steam authentication.
+	AuthId_Steam2, /**< Steam2 rendered format, ex "STEAM_1:1:4153990" */
+	AuthId_Steam3, /**< Steam3 rendered format, ex "[U:1:8307981]" */
+	AuthId_SteamID64, /**< A SteamID64 (uint64) as a String, ex "76561197968573709" */
+};
+
 /**
  * MAXPLAYERS is not the same as MaxClients.
  * MAXPLAYERS is a hardcoded value as an upper limit.  MaxClients changes based on the server.
@@ -261,14 +280,29 @@ native bool:GetClientIP(client, String:ip[], maxlen, bool:remport=true);
  * @param client		Player index.
  * @param auth			Buffer to store the client's auth string.
  * @param maxlen		Maximum length of string buffer (includes NULL terminator).
- * @param validate		Check backend validation status.
- * 				DO NOT PASS FALSE UNLESS YOU UNDERSTAND THE CONSEQUENCES,
+ * @param validate		Check backend validation status.
+ * 				DO NOT PASS FALSE UNLESS YOU UNDERSTAND THE CONSEQUENCES,
  *			        You WILL KNOW if you need to use this, MOST WILL NOT.
  * @return				True on success, false otherwise.
  * @error				If the client is not connected or the index is invalid.
  */
 native bool:GetClientAuthString(client, String:auth[], maxlen, bool:validate=true);
 
+/**
+ * Retrieves a client's authentication string (SteamID).
+ *
+ * @param client		Player index.
+ * @param authType		Auth id type and format to use.
+ * @param auth			Buffer to store the client's auth id.
+ * @param maxlen		Maximum length of string buffer (includes NULL terminator).
+ * @param validate		Check backend validation status.
+ * 				DO NOT PASS FALSE UNLESS YOU UNDERSTAND THE CONSEQUENCES,
+ *			        You WILL KNOW if you need to use this, MOST WILL NOT.
+ * @return				True on success, false otherwise.
+ * @error				If the client is not connected or the index is invalid.
+ */
+native bool:GetClientAuthId(client, AuthIdType:authType, String:auth[], maxlen, bool:validate=true);
+
 /**
  * Returns the client's Steam account ID.
  *