In his first game back after a lengthy battle with colon cancer ... Baltimore Orioles star Trey Mancini got an epic ovation from fans, teammates and opponents -- and the footage from the scene is amazing!!

Mancini -- who fought and beat the horrific disease after being diagnosed in March 2020 -- took the batter's box in the 1st inning of a spring training game against the Pirates on Sunday ... and, quickly, applause rained down on the diamond.

Fans began clapping, then Mancini's teammates joined in ... and then even Pittsburgh players jumped in -- with the Pirates catcher even standing up to praise the Baltimore slugger.

In a classy move by the umpire, he went to dust off home plate to allow the ovation to last even longer ... and then Mancini got a base hit!!!

After the game, Trey admitted it all nearly brought him to tears ... saying the ovation "meant the world to me."

"It was a really, really cool moment," the 28-year-old said, "and one of my favorite moments of my baseball career."

Mancini was forced to undergo surgery back on March 12, 2020 ... and he had very long and tough road to get through before he could return to the Orioles.

But, Trey -- after vowing to come back better than ever from the disease -- faced it all with class, and finished cancer treatments back in September.

Mancini is now fully recovered and is expected to be one of the Orioles' best players again in 2021 -- and that's something he says he's super grateful for.

"[Sunday] was a huge day for me personally," Mancini said. "Getting back in a game -- just another kind of milestone that I can check off here."