TMZ Sports has learned 53-year-old Jones Jr. will fight Robert Wilmote, AKA NDO Champ, a social media star and IFBB bodybuilding professional on January 28 in Miami.

We're told there will be five one-minute rounds.

NDO Champ (it stands for "No Days Off") is 38-years-old ... and stands 5'8". He reportedly weighs around 230 pounds.

Wilmote, who was born in Liberia and came to the U.S. as a kid to avoid civil war, is absolutely massive ... and has competed in many big bodybuilding competitions over the years, including 2nd in the Super Heavyweight division at the Arnold Classic in 2019.

Robert hasn't competed in several years but said just yesterday he planned to return to the stage soon.

He also has some fighting experience ... and has even sparred with Andre Berto.

Obviously, when it comes to the boxing experience ... he's brand new to the sport compared to RJJ -- one of the most talented pugilists to step foot in a ring.

Roy last fought in November 2020 ... in an eight-round exhibition fight against Mike Tyson. The fight was ruled a draw.