The slugger has agreed to sign with Philadelphia, sources confirmed to ESPN's Jeff Passan on Wednesday.
Harper said earlier this week that it would be a "downer" if the Phillies didn't sign Kris Bryant, Nick Castellanos or Schwarber. Philadelphia needs a left fielder, and team president Dave Dombrowski said Sunday ownership hasn't given him any limitations on spending money.
Harper said Wednesday at the Phillies spring training complex he was thrilled with the addition of Schwarber.
"It's huge for us. It gives us another bat in the lineup," Harper said. "He's a proven winner everywhere he's gone. He's done a really good job. He gives us a huge opportunity to fill that void out in left field."
The Phillies finished 82-80 last season, their first winning record since 2011, but extended their postseason drought to an NL-worst 10 years.
"I know the fans want us to be a winning team and this organization wants us to be a winning team," Harper said. "Being able to add somebody like that is going to help us that much more."
Schwarber's reputation soared in 2021 as he produced in a big way after being non-tendered by the Chicago Cubs before the season. He signed with the Washington Nationals, hitting 25 home runs in 72 games before being traded to the Boston Red Sox.
On the bigger stage of a pennant race, Schwarber produced again, compiling a .291 batting average in 41 games. The view of him as only a slugger changed in his time with Boston, where he compiled a .957 OPS and helped the Red Sox to the playoffs while being regarded as a game-changer in their lineup.
Schwarber has improved in the outfield over the course of his career and has added first base to his résumé after his stint with Boston. Long term, he'll likely be a designated hitter who can spot teammates in the outfield or at first.
Schwarber hit 94 home runs in a three-year span with the Cubs while his batting average slowly rose over time. That all came crashing down in 2020, when he hit just .188 with 11 home runs playing in 59 of 60 games. It led to his non-tender by the team that drafted him fourth overall in 2014.
Schwarber's recovery from a knee injury to participate in the 2016 World Series afforded him legendary status in Chicago but also elevated expectations. He's a classic slugger who might still have upside at the plate, as he showed in 2021.
ESPN's Jesse Rogers and The Associated Press contributed to this report.