Jason Kelce was back on the ESPN set on Monday, but it wasn't business as usual for the retired NFL star ... who opened up Monday Night Countdown with an apology for his actions.

"I think everybody's seen on social media everything that took place this week," the 36-year-old said at the top of the show from Arrowhead Stadium.

"Listen, I'm not happy with anything that took place. I'm not proud of it. In a heated moment, I chose to greet hate with hate and I just don't think that's a productive thing. I really don't."

Of course, Kelce got into it with a "fan" in Happy Valley at the Penn State vs. Ohio State football game on Saturday ... where the Eagles great used a homophobic slur three times, after Travis was called the disparaging term.

Although Kelce clearly wants to move on ... it's not all in the rearview mirror. We're told police at Penn State have launched an investigation.