diff --git a/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj b/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj
index c047c7ed71..60d90e4ba5 100644
--- a/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj
+++ b/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj
@@ -547,6 +547,7 @@
     <Compile Include="Consoles\Nintendo\NES\Boards\Mapper201.cs" />
     <Compile Include="Consoles\Nintendo\NES\Boards\Mapper202.cs" />
     <Compile Include="Consoles\Nintendo\NES\Boards\Mapper203.cs" />
+    <Compile Include="Consoles\Nintendo\NES\Boards\Mapper204.cs" />
     <Compile Include="Consoles\Nintendo\NES\Boards\Mapper218.cs" />
     <Compile Include="Consoles\Nintendo\NES\Boards\Mapper222.cs" />
     <Compile Include="Consoles\Nintendo\NES\Boards\Mapper225.cs" />
diff --git a/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/Mapper204.cs b/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/Mapper204.cs
new file mode 100644
index 0000000000..44c348d0b8
--- /dev/null
+++ b/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/Mapper204.cs
@@ -0,0 +1,61 @@
+using BizHawk.Common;
+using BizHawk.Common.NumberExtensions;
+
+namespace BizHawk.Emulation.Cores.Nintendo.NES
+{
+	// 64-in-1
+	// http://wiki.nesdev.com/w/index.php/INES_Mapper_204
+	public class Mapper204 : NES.NESBoardBase
+	{
+		private int _reg1, _reg2;
+
+		public override bool Configure(NES.EDetectionOrigin origin)
+		{
+			switch (Cart.board_type)
+			{
+				case "MAPPER204":
+					break;
+				default:
+					return false;
+			}
+
+			return true;
+		}
+
+		public override void SyncState(Serializer ser)
+		{
+			base.SyncState(ser);
+			ser.Sync("reg1", ref _reg1);
+			ser.Sync("reg2", ref _reg2);
+		}
+
+		public override void WritePRG(int addr, byte value)
+		{
+			_reg1 = addr & 0x6;
+			_reg2 = _reg1 + ((_reg1 == 0x6) ? 0 : (addr & 1));
+			_reg1 = _reg1 + ((_reg1 == 0x6) ? 1 : (addr & 1));
+
+			SetMirrorType(addr.Bit(0) ? EMirrorType.Vertical : EMirrorType.Horizontal);
+		}
+
+		public override byte ReadPRG(int addr)
+		{
+			if (addr < 0x4000)
+			{
+				return ROM[(_reg2 * 0x4000) + (addr & 0x3FFF)];
+			}
+
+			return ROM[(_reg1 * 0x4000) + (addr & 0x3FFF)];
+		}
+
+		public override byte ReadPPU(int addr)
+		{
+			if (addr < 0x2000)
+			{
+				return VROM[(_reg2 * 0x2000) + (addr & 0x1FFF)];
+			}
+
+			return base.ReadPPU(addr);
+		}
+	}
+}