Released by the Blue Jays earlier this month, the 33-year-old Ryan will first report to the Cubs' spring training complex in Mesa, Ariz. If all goes well, he'll join Triple-A Iowa.
The Cubs have only one left-handed reliever, Sean Marshall. Cubs manager Lou Piniella employed a unique strategy to keep Marshall in the game last weekend, moving him to left field when a righty reliever came in, then bringing him back to pitch one batter later.
"When they feel he's ready, we'll bring him up," Piniella said before the Cubs played at Washington. "We don't really have much left-handed depth in this organization."
The Cubs started the second half at 43-43, 3½ games behind NL Central-leading St. Louis.
Ryan was in the fourth year of a five-year, $47 million contract. He was 1-1 with a 6.53 ERA, and gave up 22 hits, including five home runs, and 17 walks in 20 2-3 innings.
Ryan spent six seasons with Baltimore and signed with Toronto after the 2005 season. He had ligament replacement surgery on his left elbow in 2007. He had 32 saves and a 2.95 ERA last year for Toronto.