Floyd Mayweather -- Rips Triple G ... You're a Nobody

Floyd Mayweather is already trading shots with Gennady Golovkin ... telling TMZ Sports the guy's just running his mouth for a payday -- and he's way out of his league.

Golovkin -- who's knocked out his last 20 opponents -- told us Mayweather would be his "dream fight" and is 100% certain he'd win if they ever got in the ring together.

Now, Mayweather's camp is issuing a response ... saying, "Everyone in boxing wants to fight Floyd. It's the biggest payday they could possibly have."

"[Golovkin] has never fought a top opponent in his whole career ... we're surprised you guys would even have him on your show to be honest."