The big showdown happened during "Monday Night Countdown" at SoFi Stadium in Los Angeles ... with the action starting well before the Rams and Dolphins kicked off their primetime matchup.

Kelce rocked a tight tank top for the contest ... and he explained he knew he was in for a tough task ahead -- but that didn't prevent him from going through with it.

"I just wanna thank everyone who had served, especially these three men right next to me who are about to kill me in a push-up contest," Kelce said.

The men had 60 seconds to do as many pushups as possible ... and Kelce was off to a hot start as his coworkers hyped him up. Inevitably, the Philadelphia Eagles legend ran out of gas.

"My chest is very pumped," Kelce said while out of breath. "It was an honor for this competition."

The winner was Seth Taylor, a veteran who served four years ... completing an insane 90 pushups. Kelce came in last with 54 ... but hey, that's pretty impressive.

Kelce was listed as 6'3", 295 pounds during his 13-year football career ... but has since taken his talents to the media.