The previous youngest hitter with a postseason grand slam was Yankees Hall of Famer Mickey Mantle, who was 21 when he accomplished the feat against the Brooklyn Dodgers in the 1953 World Series.
Buehler issued a bases-loaded walk to Braves pitcher Sean Newcomb to force in a run. Acuna took three balls and a questionable called strike before pulling a 98-mph fastball from Buehler into the seats in left-center.
The homer gave the Braves, who trailed 2-0 in the series, a 5-0 lead. The five second-inning runs were the first time Atlanta had scored in the NLDS.