New York extended its season-high winning streak to seven games by erasing a 3-0 deficit in the seventh inning. Piazza delivered the key blow in the inning, crushing a 2-0 pitch from Phillies reliever Jose Santiago over the wall in left-center field for his 14th career grand slam.
The win gave the Braves an unprecedented 11th straight division title. The Mets and Phillies figured to challenge Atlanta in the National League East this season but neither was much of a factor as the Braves became the first major league team this season to clinch a postseason berth.
Jeff D'Amico (6-10) tossed two scoreless innings in relief of rookie starter Mike Bacsik to earn the victory. Armando Benitez allowed a run in the ninth but hung on to record his 32nd save. Bacsik allowed three runs and seven hits in four-plus innings.
Philadelphia reliever Mike Timlin (4-5) was charged with five runs in the seventh before being ejected by plate umpire Larry Vanover as he was being removed from the game.
Philadelphia's Hector Mercado made his first major league start, allowing one hit over four scoreless innings and striking out four.
The Phillies were swept by the Mets in a four-game series at Veterans Stadium for the first time since 1972.