Tyler Herro Makes Kid's Day With Water Bottle Autograph, Priceless Reaction

Tyler Herro was an actual, real-life hero for at least one boy on Friday ... signing an autograph for a kid that turned the child into a puddle of joy.

The fun, heartwarming moment went down outside a school gym in Florida ... after the Miami Heat star had gotten in a workout on the court inside.

You can see in video from the scene, Herro was putting his John Hancock down for a bunch of young fans -- when, suddenly, one of the kids desperately wanted him to sign his water bottle.

The little boy grabbed it, jumped up and down -- and when Herro put his pen to it, the kid couldn't have been more thrilled.

Herro clearly loved the reaction -- 'cause even he had a big smile on his face when he returned the bottle to the boy.

Of course, Herro gets mobbed like this just about everywhere he goes in Florida -- after all, the dude is a former Sixth Man of the Year who's averaging 17.7 points per game in his career.