Bryce Harper hit a towering 413-foot grand slam in the bottom of the 9th to give the Phillies a 7-5 win over the Cubs, and a sweep of their three-game series.
The Phillies trailed 5-1 going to the 9th inning as Yu Darvish was dominant for the Cubs.
The Japanese righty threw 7 shutout innings with 10 strikeouts and just 4 hits allowed, but was taken out for a pinch-hitter in the 8th inning.
Corey Dickerson drove in the Phils' first run when his two-out single to right scored Roman Quinn, but Rhys Hoskins was thrown out at the plate trying to get another run across.
After Ranger Suarez held the Cubs in the top of the 9th, the Phils offense went to work on Rowan Wick.
Wick forced a fly out by Jean Segura to lead off the inning, but after Cesar Hernandez reached on an error, the slow bleed began for the Cubs bullpen.
Scott Kingery singled to center, and then Brad Miller did the same to bring in Hernandez with the inning's first run.
Pedro Strop took over for Wick, and he gave up a rolling single to Quinn, making the game 5-3.
Rhys Hoskins was hit by a pitch to load the bases for Harper, who would face lefty Greg Holland after a pitching change.
On a 2-2 pitch, Harper just unloaded on a drive to the second deck in right that was estimated at a 40-degree launch angle.
He sprinted around the bases, nearly running over his teammates celebrating at home plate for Harper's first career home run in the 9th inning or later against a left-handed pitcher.
It's the Phillies first walk-off grand slam with the team trailing since 1983 (Ozzie Virgil).
The Phillies open a series with San Diego on Friday.