Bodybuilding superstar Shawn Rhoden -- the reigning Mr. Olympia -- just turned himself in to authorities in Utah and pled not guilty to multiple felony rape charges, TMZ Sports has learned.

As we previously reported, Rhoden -- who goes by Flexatron -- is accused of raping a female bodybuilder during a hotel encounter back in Oct. 2018.

According to court docs, Rhoden's accuser claims Rhoden forced himself on her inside his hotel room - and despite repeated pleas to stop, he refused. She claims he eventually stopped and let her leave the room.

Authorities conducted a thorough investigation and eventually charged Rhoden with felony rape, object rape and forcible sexual assault.

Now, Rhoden's attorneys say the bodybuilder voluntarily turned himself in to authorities Tuesday  -- after negotiating his bail to be reduced from $750k to $250k.

Shawn's lawyers say he "vigorously denies the charges" and "looks forward to being cleared of these false accusations in court."

Jail records confirm Rhoden was formally arrested and booked. The 44-year-old is listed at 5'10" and 265 pounds.