UFC Legend Ken Shamrock -- I'm a Bodyguard Now ... FOR 50 CENT!!

50 Cent has SERIOUSLY upgraded his personal security team ... 'cause one of the guys watching his back is UFC Hall of Famer (and ex-WWE superstar) Ken Shamrock!!!!

49-year-old Shamrock was spotted on the rapper's personal protection detail at the CES convection in Las Vegas yesterday ... and afterward, Ken confirmed he's now working for a company that does private security for several "high-end clients," including Fiddy.

After the convention, Ken tweeted that it was a "pleasure" working for 50 ... and said the rapper is, "By far one of the classiest gents I have ever worked with."

As for Ken's fighting career -- he may not have given up on the ring entirely ... just last month, he appeared in a wrestling event in Costa Rica.

But for now, don't screw with Mr. Cent ... or you might end up in a nasty ankle lock.