On March 2, 2019, Harper signed a 13-year, $330 million contract with the Philadelphia Phillies.[97][98] This deal set the record for the largest MLB contract until Mike Trout signed a 12-year, $430 million contract extension with the Los Angeles Angels.[99] Harper chose to wear No. 3 with the club, as the number he wore with the Nationals (34) was being considered for retirement by the Phillies in honor of the late Roy Halladay.[100] At his first press conference, Harper misspoke, saying, "We want to bring a title back to D.C." The Nationals won the 2019 World Series in Harper's absence.
Harper's first hit as a member of the Phillies was a 465-foot home run to the second deck of the right field bleachers at Citizens Bank Park on March 30, 2019, off Jesse Biddle of the Atlanta Braves. On August 15, 2019, Harper hit a walk-off grand slam against the Chicago Cubs. On September 3, 2019, Harper would bat his 100th RBI of the season for the second consecutive year and was the first Philadelphia Phillies player since Ryan Howard in 2011 to hit for 100 RBIs or more in a season. However, the Phillies missed the playoffs, being eliminated officially on September 24 by losing to the eventual World Series champions and Harper's former team, the Washington Nationals.