Conor McGregor has returned to the U.S. -- he's got a fight here in 2 weeks -- and he's already hitting the ground running ... literally.

"Out running on the streets of Las Vegas today," McGregor said over the weekend ... "We are two weeks from fight night!"

"So great to be back on U.S soil."

Of course, McGregor is taking on Cowboy Cerrone at UFC 246 on January 18 at the T-Mobile Arena in Vegas in his first fight since losing to Khabib Nurmagomedov in Oct. 2018.

But, he's not coasting into the fight -- McGregor's been training since his arrival. In fact, Conor ran into UFC star Dominick Cruz during a session at the UFC Performance Institute.

Before this trip, we hadn't seen Conor in America since around the time he was being prosecuted for stomping on a cell phone outside of a Miami hotel back in March.

The Irishman pled not guilty and the charges were ultimately dropped after the victim stopped cooperating with police ... but it wasn't a good look for Conor.

Since then, CM's had other problems ... he punched a man in a bar in Ireland for no good reason during an incident in April and reportedly been investigated for 2 alleged sexual assaults in his home country. He adamantly DENIES the allegations in both cases.