Jason Kelce lost it at a person in Pennsylvania ... grabbing someone's phone and throwing it to the ground before storming off -- right after someone in the crowd called Travis a gay slur.

The former NFL star-turned-commentator was in Happy Valley for the matchup between Ohio State and Penn State Saturday morning, and a crowd started following the star ... cheering him on and asking for fist bumps at first.

It didn't stay positive for long though ... 'cause one troll decided to blast Jason's bro Travis using a slur. You can listen to the video for yourself -- it's an aggressive attack on the tight end and his GF, Taylor Swift.

At this point, Kelce turns around and gets in a person's face before grabbing the phone and chucking it to the ground. You can't see the phone after he throws it down ... but, the noise it makes when it hits the ground screams "shattered."

Worth noting ... no video has surfaced yet that shows the person who yells the slur -- so, it's unclear if Jason threw the correct person's phone down. Another angle seems to show the man yelling at Jason right before Kelce takes his phone and smashes it, but unclear if he also yelled the slur.

Jason usually has pretty genial interactions with fans ... but, this one definitely set him off. We've reached out to Jason and ESPN, which is the company Jason works for, about the video -- so far, no word back.