Herschel Walker -- I'm 51 Years Old ... and I Want to FIGHT AGAIN!!!

Herschel Walker is UNDEFEATED in the world of MMA ... but the 51-year-old former NFL superstar tells TMZ he wants to get in the ring one more time ... and it could happen before the end of the year.

Walker famously signed a deal with Strikeforce back in 2009 ... and proceeded to annihilate his first two opponents.

But this week, the former Heisman Trophy winner told us he's still got gas in the tank ... and he ain't ready to hang up his gloves just yet.

Fun Fact -- Herschel isn't just a stellar athlete, he's also a stellar businessman who owns a very successful food company called Renaissance Man Food Services.

According to Herschel, RMFS is "one of the largest in the United States" ... and primarily deals with chicken.