The Athletic's Ken Rosenthal reports that the deal is for one year and $10.5 million, pending a physical.
Chapman joins the Pirates after a half-season with the World Series champion Texas Rangers. Chapman joined the Rangers via a midseason trade from the Kansas City Royals. He made 30 appearances with the Rangers, posting a 3.72 ERA and 1.276 WHIP with 50 strikeouts and 16 walks allowed in 29 innings pitched. He tallied four saves while working as a part-time closer alongside fellow reliever José Leclerc and primary closer Will Smith.
In nine postseason appearances, Chapman posted a 2.25 ERA with six strikeouts and five walks allowed in eight innings en route to the World Series victory. He allowed one earned run in two World Series appearances totaling 1 2/3 innings.
Chapman, 35, has pitched 14 MLB seasons. He earned seven All-Star appearances in his prime as one of baseball's most dominant closers with the Cincinnati Reds and New York Yankees, plus a run to the World Series with the Chicago Cubs.
Chapman made his last All-Star appearance in 2021, but remains one of the hardest throwers in baseball, with a 99.5 mph average fastball velocity that ranks in the 99th percentile in MLB.
He'll join a Pirates team that finished in fourth in the NL Central with a 76-86 record in 2023. That marked the first time since 2018 that the Pirates won more than 70 games.
The Pirates are looking to build on that progress and hope Chapman can help them to their first winning record in six seasons. He'll join a bullpen that features two-time All-Star closer David Bednar, who tied for the NL lead with 39 saves last season.