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 of 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.