A Citi Field security guard was forced to leave a game on Wednesday afternoon ... after he was drilled on the top of the head by a towering foul ball.
The scary moment just happened in the second inning of the Mets vs. Braves game in New York ... when Atlanta catcher Travis D'Arnaud hit a pitch way into the air down the third-base line.
As the ball was coming down in foul ground, though, a security guard couldn't track it -- and it ended up plunking him.
The ball hit the man so hard, D'Arnaud visibly winced at the sight.