TMZ shared footage of the Irishman sparring in a boxing ring, taking part in some striking and movement drills:
In a separate report, TMZ said the clip came from a session on Wednesday at the Fight Club Gym.
The footage emerged just a couple of days on from McGregor's retirement tweet, in which he said he was done with the sport:
Conor McGregor @TheNotoriousMMA
Hey guys quick announcement, I’ve decided to retire from the sport formally known as “Mixed Martial Art” today. I wish all my old colleagues well going forward in competition. I now join my former partners on this venture, already in retirement. Proper Pina Coladas on me fellas!
The retirement announcement came on the same day Tariq Panja of the New York Times reported McGregor is being investigated by Irish police after a woman said he sexually assaulted her in a Dublin hotel in December.
McGregor previously said he would leave the Octagon in April 2016. "I have decided to retire young. Thanks for the cheese. Catch ya's later," he said in a Twitter post. He returned to the Octagon in August of that year.
McGregor was last in action at UFC 229, when he was forced to submit in a showdown with Khabib Nurmagomedov.
The fight came after an ill-tempered buildup, with McGregor attacking a bus on which Nurmagomedov was on board at UFC 223. The Irishman avoided a jail sentence for his actions, after pleading guilty to disorderly conduct.
After the bout with Nurmagomedov, a brawl in the Octagon between members of both men's camps resulted in a six-month suspension for McGregor and a nine-month ban for his opponent.