MLB star Hunter Greene uncorked a fastball on Tuesday night ... and then he unloaded the contents of his stomach -- vomiting all over the mound right in the middle of the Reds' game.
The nasty moment happened just minutes after the ace took the bump for Cincinnati's tilt with the Pirates at Great American Ball Park.
Greene walked the first batter he faced, and then in the ensuing at-bat, he threw up everywhere after making a pitch.
Check out the video from the game's broadcast, just moments after the ball hit the catcher's mitt .