A person familiar with the negotiations says side-arming reliever Steve Cishek and the Cubs have agreed to a two-year contract.
The person spoke to The Associated Press on condition of anonymity Thursday because the deal was subject to a successful physical and had not been announced.
A 31-year-old right-hander, Cishek had a 2.01 ERA and a 3-2 record in 49 relief appearances this year for Seattle and Tampa Bay, which acquired him on July 28. He would join rejoin a rebuilt Cubs bullpen that includes right-hander Brandon Morrow, who finalized a $21 million, two-year contract on Tuesday.
Cishek is 24-28 with a 2.73 ERA and 121 saves over eight seasons.
His eight-year major league career started with the Florida and Miami Marlins from 2010-15. He was traded to St. Louis in July 2015, became a free agent and signed a $10 million, two-year contract with the Mariners.
"You got to have that great bullpen to play the last game of the year and win it," Cubs manager Joe Maddon said.
Also, Stephen Piscotty has been traded by the St. Louis Cardinals to the Oakland Athletics for two infield prospects, allowing the outfielder to be near his family's home in Pleasanton, California, following his mother's diagnosis with Lou Gehrig's disease.
Gretchen Piscotty was diagnosed in May with amyotrophic lateral sclerosis, a disease that attacks nerve cells. Her son left the Cardinals for five days after the diagnosis and returned May 31.
"You are never making a player trade simply for geographic or sentimental reasons," John Mozeliak, president of baseball operations for the Cardinals, told St. Louis media. "It had to be something that made sense for us. There were certainly some opportunities to move him elsewhere. When you are looking at how to break a tie, clearly that did play into it."
St. Louis receives Yairo Munoz and Max Schrock in the deal announced Thursday.
The 26-year-old Piscotty hit .235 with nine homers and 39 RBIs in 107 games last season, spending stretches on the disabled list because of a strained hamstring and groin. He was selected by St. Louis from Stanford with the 36th overall pick in the 2012 amateur draft, made his big league debut in July 2015.
Piscotty hit 22 home runs with 85 RBIs in 2016, and has batted .268 with 38 homers and 163 RBIs in 2½ major league seasons.
Piscotty agreed in May to a $33.5 million, six-year contract.
Munoz, 22, hit .300 with 13 homers and 68 RBIs this year for Double-A Midland and Triple-A Nashville. Schrock, 23, batted .321 with seven homers and 46 RBIs for Midland, and was a Texas League All-Star.