diff --git a/backends/firrtl/firrtl.cc b/backends/firrtl/firrtl.cc
index 40d05a036..ff1329532 100644
--- a/backends/firrtl/firrtl.cc
+++ b/backends/firrtl/firrtl.cc
@@ -392,7 +392,37 @@ 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<string> port_decls;
+
+		for (auto wire : module->wires())
+		{
+			const auto wireName = make_id(wire->name);
+			std::string wireFileinfo = getFileinfo(wire);
+
+			// Maybe not needed?
+			if (wire->port_id)
+			{
+				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());
@@ -1076,6 +1106,22 @@ 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();
+		}
 	}
 };