The Phillies sent minor league outfielders Mickey Moniak and Jadiel Sanchez to the Los Angeles Angels for Syndergaard, who returns to the NL East after spending his first six seasons with the New York Mets.
The 29-year-old Syndergaard went 5-8 with a 3.83 ERA in 15 starts after agreeing to a $21 million, one-year deal with Los Angeles in November.
"I'm excited to have the opportunity to play games for a team that is in the postseason hunt," Syndergaard said in the Angels clubhouse after being informed about the trade. "It's going to be an exciting race. I'm not going to be able to avoid the New York Mets media now."
He will get his first start for the Phillies on Thursday, when he faces the Washington Nationals.
Moniak gets a fresh start with a new organization after he was selected by Philadelphia with the No. 1 overall pick in the 2016 amateur draft. The 24-year-old Moniak is a .129 hitter in 47 career games.
Philadelphia sent minor league pitcher Ben Brown to the Chicago Cubs for the 37-year-old Robertson, one of the top relievers on the market ahead of the trade deadline. The Phillies got Marsh from the Angels in exchange for catching prospect Logan O'Hoppe.
Philadelphia is fighting for one of three National League wild cards. It hasn't made the postseason since 2011.
The well-traveled Robertson also played for the Phillies in 2019, making seven appearances before he was sidelined by an elbow injury that required surgery. He didn't make it back to the majors until he pitched in 12 games with Tampa Bay in 2021.
Robertson is 3-0 with a 2.23 ERA and 14 saves in 36 appearances this year. The right-hander finalized a $3.5 million, one-year contract with the Cubs on March 16.
"He was a treat to manage. He influenced a lot of guys in a positive way," Cubs manager David Ross said. "He's very professional. Experience is a big thing in this game. He's been around. He understands."
Robertson made his big league debut in 2008 and spent his first seven seasons with the New York Yankees, winning the World Series in 2009. He has 151 saves and a 2.89 ERA in 709 career appearances.
The 22-year-old Brown was selected by the Phillies in the 33rd round of the 2017 amateur draft. The right-hander was 3-5 with a 3.08 ERA in 16 appearances with High-A Jersey Shore this year, including 15 starts.
Marsh is a lefty-hitting, righty-throwing 24-year-old with speed and power projection. He ranked among Los Angeles' top prospects before debuting in the majors last season, but he has struggled in the big leagues. He's a .239 hitter in 163 games with 10 homers, 14 stolen bases and a .653 OPS.
His defense is well regarded, and that's an area Philadelphia sorely needed to upgrade. Marsh trails only Brett Phillips and Minnesota's Max Kepler among major league outfielders in Outs Above Average, according to Statcast.
"It was a little bit of an eye opener, but I gotta keep keep going and so does this organization. It's just part of the business," Marsh said.
The Phillies have stayed in the playoff race despite losing 2021 NL MVP Bryce Harper because of a broken thumb on June 25. He's expected to begin swinging a bat soon and should return before the end of the season.
Philadelphia also designated veteran outfielder Odúbel Herrera and reliever Jeurys Familia for assignment. The move could end Herrera's eight-year tenure with the team that included a lengthy ban under Major League Baseball's joint domestic violence policy.
The 22-year-old O'Hoppe has developed into one of the majors' top catching prospects. He was batting .275 with 15 homers and 45 RBIs in 75 games with Double-A Reading.