diff --git a/Common/UI/View.cpp b/Common/UI/View.cpp
index 18c573fcb2..3dcd94ef10 100644
--- a/Common/UI/View.cpp
+++ b/Common/UI/View.cpp
@@ -852,6 +852,11 @@ void CheckBox::GetContentDimensions(const UIContext &dc, float &w, float &h) con
 	w = bounds_.w;
 }
 
+BitCheckBox::BitCheckBox(uint32_t *bitfield, uint32_t bit, std::string_view text, std::string_view smallText, LayoutParams *layoutParams)
+	: CheckBox(nullptr, text, smallText, layoutParams), bitfield_(bitfield), bit_(bit) {
+	_dbg_assert_msg_(bit != 0, "bit is a mask, not a bit index");
+}
+
 void BitCheckBox::Toggle() {
 	if (bitfield_) {
 		*bitfield_ = *bitfield_ ^ bit_;
diff --git a/Common/UI/View.h b/Common/UI/View.h
index 2372d1ad06..42af14edab 100644
--- a/Common/UI/View.h
+++ b/Common/UI/View.h
@@ -916,10 +916,8 @@ private:
 
 class BitCheckBox : public CheckBox {
 public:
-	BitCheckBox(uint32_t *bitfield, uint32_t bit, std::string_view text, std::string_view smallText = "", LayoutParams *layoutParams = nullptr)
-		: CheckBox(nullptr, text, smallText, layoutParams), bitfield_(bitfield), bit_(bit) {
-	}
-    
+	// bit is a bitmask (should only have a single bit set), not a bit index.
+	BitCheckBox(uint32_t *bitfield, uint32_t bit, std::string_view text, std::string_view smallText = "", LayoutParams *layoutParams = nullptr);
 	BitCheckBox(int *bitfield, int bit, std::string_view text, std::string_view smallText = "", LayoutParams *layoutParams = nullptr) : BitCheckBox((uint32_t *)bitfield, (uint32_t)bit, text, smallText, layoutParams) {}
 
 	void Toggle() override;
diff --git a/Core/Config.cpp b/Core/Config.cpp
index ae19a040e0..6bdbf8e06c 100644
--- a/Core/Config.cpp
+++ b/Core/Config.cpp
@@ -306,6 +306,8 @@ static const ConfigSetting generalSettings[] = {
 	ConfigSetting("IgnoreCompatSettings", &g_Config.sIgnoreCompatSettings, "", CfgFlag::PER_GAME | CfgFlag::REPORT),
 
 	ConfigSetting("RunBehindPauseMenu", &g_Config.bRunBehindPauseMenu, false, CfgFlag::DEFAULT),
+
+	ConfigSetting("ShowGPOLEDs", &g_Config.bShowGPOLEDs, false, CfgFlag::PER_GAME),
 };
 
 static bool DefaultSasThread() {
diff --git a/Core/Config.h b/Core/Config.h
index f6addf3f1c..61a30c5d5a 100644
--- a/Core/Config.h
+++ b/Core/Config.h
@@ -265,6 +265,9 @@ public:
 	bool bRenderDuplicateFrames;
 	bool bRenderMultiThreading;
 
+	// HW debug
+	bool bShowGPOLEDs;
+
 	// Sound
 	bool bEnableSound;
 	int iAudioBackend;
diff --git a/Core/HLE/sceKernel.cpp b/Core/HLE/sceKernel.cpp
index 28d6a60a0b..d4da98bf42 100644
--- a/Core/HLE/sceKernel.cpp
+++ b/Core/HLE/sceKernel.cpp
@@ -100,6 +100,8 @@ static bool kernelRunning = false;
 KernelObjectPool kernelObjects;
 KernelStats kernelStats;
 u32 registeredExitCbId;
+u32 g_GPOBits;  // Really just 8 bits on the real hardware.
+u32 g_GPIBits;  // Really just 8 bits on the real hardware.
 
 void __KernelInit()
 {
@@ -161,6 +163,7 @@ void __KernelInit()
 	__PPGeInit();
 
 	kernelRunning = true;
+	g_GPOBits = 0;
 	INFO_LOG(SCEKERNEL, "Kernel initialized.");
 }
 
@@ -355,18 +358,19 @@ int sceKernelRegisterDefaultExceptionHandler()
 	return 0;
 }
 
-void sceKernelSetGPO(u32 ledAddr)
+void sceKernelSetGPO(u32 ledBits)
 {
-	// Sets debug LEDs.
-	// Not really interesting, and a few games really spam it
-	// DEBUG_LOG(SCEKERNEL, "sceKernelSetGPO(%02x)", ledAddr);
+	// Sets debug LEDs. Some games do interesting stuff with this, like a metronome in Parappa.
+	// Shows up as a vertical strip of LEDs at the side of the screen, if enabled.
+	g_GPOBits = ledBits;
 }
 
 u32 sceKernelGetGPI()
 {
 	// Always returns 0 on production systems.
-	DEBUG_LOG(SCEKERNEL, "0=sceKernelGetGPI()");
-	return 0;
+	// On developer systems, there are 8 switches that control the lower 8 bits of the return value.
+	DEBUG_LOG(SCEKERNEL, "%d=sceKernelGetGPI()", g_GPIBits);
+	return g_GPIBits;
 }
 
 // #define LOG_CACHE
diff --git a/Core/HLE/sceKernel.h b/Core/HLE/sceKernel.h
index 722f89d37d..4c03d5848b 100644
--- a/Core/HLE/sceKernel.h
+++ b/Core/HLE/sceKernel.h
@@ -569,6 +569,9 @@ struct KernelStats {
 extern KernelStats kernelStats;
 extern u32 registeredExitCbId;
 
+extern u32 g_GPOBits;
+extern u32 g_GPIBits;
+
 void Register_ThreadManForUser();
 void Register_ThreadManForKernel();
 void Register_LoadExecForUser();
diff --git a/UI/DevScreens.cpp b/UI/DevScreens.cpp
index 8da4308886..f16dbfef01 100644
--- a/UI/DevScreens.cpp
+++ b/UI/DevScreens.cpp
@@ -56,6 +56,7 @@
 #include "Core/System.h"
 #include "Core/Reporting.h"
 #include "Core/CoreParameter.h"
+#include "Core/HLE/sceKernel.h"  // GPI/GPO
 #include "Core/MIPS/MIPSTables.h"
 #include "Core/MIPS/JitCommon/JitBlockCache.h"
 #include "Core/MIPS/JitCommon/JitCommon.h"
@@ -141,6 +142,11 @@ void DevMenuScreen::CreatePopupContents(UI::ViewGroup *parent) {
 
 	items->Add(new Choice(dev->T("Reset limited logging")))->OnClick.Handle(this, &DevMenuScreen::OnResetLimitedLogging);
 
+	items->Add(new Choice(dev->T("GPI/GPO switches/LEDs")))->OnClick.Add([=](UI::EventParams &e) {
+		screenManager()->push(new GPIGPOScreen(dev->T("GPI/GPO switches/LEDs")));
+		return UI::EVENT_DONE;
+	});
+
 	items->Add(new Choice(dev->T("Create frame dump")))->OnClick.Add([](UI::EventParams &e) {
 		GPURecord::RecordNextFrame([](const Path &dumpPath) {
 			NOTICE_LOG(SYSTEM, "Frame dump created at '%s'", dumpPath.c_str());
@@ -213,6 +219,16 @@ void DevMenuScreen::dialogFinished(const Screen *dialog, DialogResult result) {
 	// TriggerFinish(DR_OK);
 }
 
+void GPIGPOScreen::CreatePopupContents(UI::ViewGroup *parent) {
+	using namespace UI;
+	auto dev = GetI18NCategory(I18NCat::DEVELOPER);
+	parent->Add(new CheckBox(&g_Config.bShowGPOLEDs, dev->T("Show GPO LEDs")));
+	for (int i = 0; i < 8; i++) {
+		std::string name = ApplySafeSubstitutions(dev->T("GPI switch %1"), i);
+		parent->Add(new BitCheckBox(&g_GPIBits, 1 << i, name));
+	}
+}
+
 void LogScreen::UpdateLog() {
 	using namespace UI;
 	RingbufferLogListener *ring = LogManager::GetInstance()->GetRingbufferListener();
diff --git a/UI/DevScreens.h b/UI/DevScreens.h
index 29fbf5690b..9ae176a840 100644
--- a/UI/DevScreens.h
+++ b/UI/DevScreens.h
@@ -145,6 +145,15 @@ private:
 	unsigned int addr_;
 };
 
+class GPIGPOScreen : public PopupScreen {
+public:
+	GPIGPOScreen(std::string_view title) : PopupScreen(title, "OK") {}
+	const char *tag() const override { return "GPIGPO"; }
+
+protected:
+	void CreatePopupContents(UI::ViewGroup *parent) override;
+};
+
 class JitCompareScreen : public UIDialogScreenWithBackground {
 public:
 	void CreateViews() override;
diff --git a/UI/EmuScreen.cpp b/UI/EmuScreen.cpp
index da7d4f278f..eafa4c9fde 100644
--- a/UI/EmuScreen.cpp
+++ b/UI/EmuScreen.cpp
@@ -1521,6 +1521,8 @@ bool EmuScreen::hasVisibleUI() {
 		return true;
 	if (g_Config.bEnableCardboardVR || g_Config.bEnableNetworkChat)
 		return true;
+	if (g_Config.bShowGPOLEDs)
+		return true;
 	// Debug UI.
 	if ((DebugOverlay)g_Config.iDebugOverlay != DebugOverlay::OFF || g_Config.bShowDeveloperMenu)
 		return true;
@@ -1571,6 +1573,26 @@ void EmuScreen::renderUI() {
 	}
 #endif
 
+	if (g_Config.bShowGPOLEDs) {
+		// Draw a vertical strip of LEDs at the right side of the screen.
+		const float ledSize = 24.0f;
+		const float spacing = 4.0f;
+		const float height = 8 * ledSize + 7 * spacing;
+		const float x = ctx->GetBounds().w - spacing - ledSize;
+		const float y = (ctx->GetBounds().h - height) * 0.5f;
+		ctx->FillRect(UI::Drawable(0xFF000000), Bounds(x - spacing, y - spacing, ledSize + spacing * 2, height + spacing * 2));
+		for (int i = 0; i < 8; i++) {
+			int bit = (g_GPOBits >> i) & 1;
+			uint32_t color = 0xFF30FF30;
+			if (!bit) {
+				color = darkenColor(darkenColor(color));
+			}
+			Bounds ledBounds(x, y + (spacing + ledSize) * i, ledSize, ledSize);
+			ctx->FillRect(UI::Drawable(color), ledBounds);
+		}
+		ctx->Flush();
+	}
+
 	if (coreState == CORE_RUNTIME_ERROR || coreState == CORE_STEPPING) {
 		const MIPSExceptionInfo &info = Core_GetExceptionInfo();
 		if (info.type != MIPSExceptionType::NONE) {
diff --git a/UI/GameSettingsScreen.cpp b/UI/GameSettingsScreen.cpp
index 40283066fe..5dc9404dd7 100644
--- a/UI/GameSettingsScreen.cpp
+++ b/UI/GameSettingsScreen.cpp
@@ -1843,6 +1843,11 @@ void DeveloperToolsScreen::CreateViews() {
 
 	list->Add(new CheckBox(&g_Config.bShowOnScreenMessages, dev->T("Show on-screen messages")));
 
+	list->Add(new Choice(dev->T("GPI/GPO switches/LEDs")))->OnClick.Add([=](UI::EventParams &e) {
+		screenManager()->push(new GPIGPOScreen(dev->T("GPI/GPO switches/LEDs")));
+		return UI::EVENT_DONE;
+	});
+
 #if PPSSPP_PLATFORM(ANDROID)
 	static const char *framerateModes[] = { "Default", "Request 60Hz", "Force 60Hz" };
 	PopupMultiChoice *framerateMode = list->Add(new PopupMultiChoice(&g_Config.iDisplayFramerateMode, gr->T("Framerate mode"), framerateModes, 0, ARRAY_SIZE(framerateModes), I18NCat::GRAPHICS, screenManager()));