Terms were not disclosed.
The 35-year-old Soriano began the 2014 season as the closer for the Washington Nationals and converted 22 of 24 save opportunities with an 0.97 ERA before the All-Star break. After that, though, he blew five of 15 save chances, and he lost the job to Drew Storen.
Soriano has 207 career saves over 13 seasons. He also has previously pitched for Seattle, Atlanta, Tampa Bay and the New York Yankees.
The contract could lead to a reunion for Soriano and Joe Maddon, who was the manager in Tampa Bay when the All-Star right-hander had 45 saves for the Rays in 2010. Soriano has three seasons with at least 40 saves.
"Had a great time with him," Maddon said over the weekend before the Cubs signed Soriano. "Had a great season down there. He's very good."
Soriano recently hired agent Alan Nero, who also represents Maddon.
The Cubs have recently been using a closer-by-committee approach with Hector Rondon, Jason Motte and Pedro Strop. All three earned saves in last weekend's series against the Nationals.
The Cubs' bullpen ranks 20th in the majors with a 3.68 ERA, having allowed 67 earned runs while going 12-10 and earning just 14 saves.
Jesse Rogers of ESPNChicago.com and The Associated Press contributed to this report.
Cubs turn to Rafael Soriano for bullpen depth
ESPN SweetSpot blogger David Schoenfield discusses the Cubs' signing of free-agent reliever Rafael Soriano.