diff --git a/UI/data/locale/en-US.ini b/UI/data/locale/en-US.ini
index cc9001a40..ab3592148 100644
--- a/UI/data/locale/en-US.ini
+++ b/UI/data/locale/en-US.ini
@@ -72,6 +72,11 @@ RemuxRecordings="Remux Recordings"
 Next="Next"
 Back="Back"
 
+# warning if program already open
+AlreadyRunning.Title="OBS is already running"
+AlreadyRunning.Text="OBS is already running!  Unless you meant to do this, please shut down any existing instances of OBS before trying to run a new instance.  If you have OBS set to minimize to the system tray, please check to see if it's still running there."
+AlreadyRunning.LaunchAnyway="Launch Anyway"
+
 # copy filters
 Copy.Filters="Copy Filters"
 Paste.Filters="Paste Filters"
diff --git a/UI/obs-app.cpp b/UI/obs-app.cpp
index 25d3e4b5a..3f725e3a2 100644
--- a/UI/obs-app.cpp
+++ b/UI/obs-app.cpp
@@ -61,7 +61,7 @@ static log_handler_t def_log_handler;
 static string currentLogFile;
 static string lastLogFile;
 
-static bool portable_mode = false;
+bool portable_mode = false;
 static bool log_verbose = false;
 static bool unfiltered_log = false;
 bool opt_start_streaming = false;
@@ -1327,6 +1327,46 @@ static int run_program(fstream &logFile, int argc, char *argv[])
 
 		program.installTranslator(&translator);
 
+#ifdef _WIN32
+		/* --------------------------------------- */
+		/* check and warn if already running       */
+
+		bool already_running = false;
+		RunOnceMutex rom = GetRunOnceMutex(already_running);
+
+		if (already_running) {
+			blog(LOG_WARNING, "\n================================");
+			blog(LOG_WARNING, "Warning: OBS is already running!");
+			blog(LOG_WARNING, "================================\n");
+
+			QMessageBox::StandardButtons buttons(
+					QMessageBox::Yes | QMessageBox::Cancel);
+			QMessageBox mb(QMessageBox::Question,
+					QTStr("AlreadyRunning.Title"),
+					QTStr("AlreadyRunning.Text"),
+					buttons,
+					nullptr);
+			mb.setButtonText(QMessageBox::Yes,
+					QTStr("AlreadyRunning.LaunchAnyway"));
+			mb.setButtonText(QMessageBox::Cancel, QTStr("Cancel"));
+			mb.setDefaultButton(QMessageBox::Cancel);
+
+			QMessageBox::StandardButton button;
+			button = (QMessageBox::StandardButton)mb.exec();
+			if (button == QMessageBox::Cancel) {
+				blog(LOG_INFO, "User shut down the program "
+						"because OBS was already "
+						"running");
+				return 0;
+			}
+
+			blog(LOG_WARNING, "User is now running a secondary "
+					"instance of OBS!");
+		}
+
+		/* --------------------------------------- */
+#endif
+
 		if (!program.OBSInit())
 			return 0;
 
diff --git a/UI/obs-app.hpp b/UI/obs-app.hpp
index acbb9ae06..53043ac3f 100644
--- a/UI/obs-app.hpp
+++ b/UI/obs-app.hpp
@@ -176,6 +176,7 @@ static inline int GetProfilePath(char *path, size_t size, const char *file)
 	return window->GetProfilePath(path, size, file);
 }
 
+extern bool portable_mode;
 extern bool opt_start_streaming;
 extern bool opt_start_recording;
 extern bool opt_start_replaybuffer;
diff --git a/UI/platform-windows.cpp b/UI/platform-windows.cpp
index 6675aa664..524361b07 100644
--- a/UI/platform-windows.cpp
+++ b/UI/platform-windows.cpp
@@ -34,6 +34,7 @@ using namespace std;
 #include <mmdeviceapi.h>
 #include <audiopolicy.h>
 
+#include <util/windows/WinHandle.hpp>
 #include <util/windows/HRError.hpp>
 #include <util/windows/ComPtr.hpp>
 
@@ -271,3 +272,65 @@ uint64_t CurrentMemoryUsage()
 
 	return (uint64_t)pmc.WorkingSetSize;
 }
+
+struct RunOnceMutexData {
+	WinHandle handle;
+
+	inline RunOnceMutexData(HANDLE h) : handle(h) {}
+};
+
+RunOnceMutex::RunOnceMutex(RunOnceMutex &&rom)
+{
+	delete data;
+	data = rom.data;
+	rom.data = nullptr;
+}
+
+RunOnceMutex::~RunOnceMutex()
+{
+	delete data;
+}
+
+RunOnceMutex &RunOnceMutex::operator=(RunOnceMutex &&rom)
+{
+	delete data;
+	data = rom.data;
+	rom.data = nullptr;
+	return *this;
+}
+
+RunOnceMutex GetRunOnceMutex(bool &already_running)
+{
+	string name;
+
+	if (!portable_mode) {
+		name = "OBSStudioCore";
+	} else {
+		char path[500];
+		*path = 0;
+		GetConfigPath(path, sizeof(path), "");
+		name = "OBSStudioPortable";
+		name += path;
+	}
+
+	BPtr<wchar_t> wname;
+	os_utf8_to_wcs_ptr(name.c_str(), name.size(), &wname);
+
+	if (wname) {
+		wchar_t *temp = wname;
+		while (*temp) {
+			if (!iswalnum(*temp))
+				*temp = L'_';
+			temp++;
+		}
+	}
+
+	HANDLE h = OpenMutexW(SYNCHRONIZE, false, wname.Get());
+	already_running = !!h;
+
+	if (!already_running)
+		h = CreateMutexW(nullptr, false, wname.Get());
+
+	RunOnceMutex rom(h ? new RunOnceMutexData(h) : nullptr);
+	return rom;
+}
diff --git a/UI/platform.hpp b/UI/platform.hpp
index 50efef300..ca0780b48 100644
--- a/UI/platform.hpp
+++ b/UI/platform.hpp
@@ -44,6 +44,22 @@ void SetProcessPriority(const char *priority);
 void SetWin32DropStyle(QWidget *window);
 bool DisableAudioDucking(bool disable);
 uint64_t CurrentMemoryUsage();
+
+struct RunOnceMutexData;
+
+class RunOnceMutex {
+	RunOnceMutexData *data = nullptr;
+public:
+	RunOnceMutex(RunOnceMutexData *data_) : data(data_) {}
+	RunOnceMutex(const RunOnceMutex &rom) = delete;
+	RunOnceMutex(RunOnceMutex &&rom);
+	~RunOnceMutex();
+
+	RunOnceMutex &operator=(const RunOnceMutex &rom) = delete;
+	RunOnceMutex &operator=(RunOnceMutex &&rom);
+};
+
+RunOnceMutex GetRunOnceMutex(bool &already_running);
 #endif
 
 #ifdef __APPLE__