Cahill, 27, drew interest from multiple clubs this winter, including the Pirates. He spent the second half of last season with the Cubs, posting a solid 2.12 ERA in 17 innings of relief work.
Cahill joins right-handers Hector Rondon, Pedro Strop and Justin Grimm in a Cubs' bullpen that has already added left-hander Rex Brothers this winter. He will likely see time as a long-man or middle reliever, with the ability to spot-start due to his experience as a starter at the major-league level.
In parts of seven major-league seasons with the Athletics (2009-11), Diamondbacks (2012-14), Braves (2015) and Cubs (2015), Cahill is the owner of a lifetime 4.13 ERA in 212 appearances (172 starts). He was an All-Star in 2010 with Oakland.