diff --git a/backends/firrtl/firrtl.cc b/backends/firrtl/firrtl.cc
index 89df0366f..a90b0b87a 100644
--- a/backends/firrtl/firrtl.cc
+++ b/backends/firrtl/firrtl.cc
@@ -392,7 +392,34 @@ struct FirrtlWorker
 		return result;
 	}
 
-	void run()
+	void emit_extmodule()
+	{
+		std::string moduleFileinfo = getFileinfo(module);
+		f << stringf("  extmodule %s: %s\n", make_id(module->name), moduleFileinfo.c_str());
+		vector<std::string> port_decls;
+
+		for (auto wire : module->wires())
+		{
+			const auto wireName = make_id(wire->name);
+			std::string wireFileinfo = getFileinfo(wire);
+
+			if (wire->port_input && wire->port_output)
+			{
+				log_error("Module port %s.%s is inout!\n", log_id(module), log_id(wire));
+			}
+			port_decls.push_back(stringf("    %s %s: UInt<%d> %s\n", wire->port_input ? "input" : "output",
+					wireName, wire->width, wireFileinfo.c_str()));
+		}
+
+		for (auto &str : port_decls)
+		{
+			f << str;
+		}
+
+		f << stringf("\n");
+	}
+
+	void emit_module()
 	{
 		std::string moduleFileinfo = getFileinfo(module);
 		f << stringf("  module %s: %s\n", make_id(module->name), moduleFileinfo.c_str());
@@ -1078,6 +1105,18 @@ struct FirrtlWorker
 
 		for (auto str : wire_exprs)
 			f << str;
+
+		f << stringf("\n");
+	}
+
+	void run()
+	{
+		// Blackboxes should be emitted as `extmodule`s in firrtl. Only ports are
+		// emitted in such a case.
+		if (module->get_blackbox_attribute())
+			emit_extmodule();
+		else
+			emit_module();
 	}
 };