Hunter was 2 -2 with a 3.63 ERA in 39 appearances for Baltimore this season. The 29-year-old right-hander is 44-33 with a 4.28 ERA in eight seasons with Texas and the Orioles. He won 13 games as a starter with the Rangers in 2010 and was traded to Baltimore the following season.
Since moving into a full-time bullpen role with Baltimore in 2013, Hunter is 11-9 with 15 saves and a 3.05 ERA.
Since joining the Orioles, Hunter proved to be a light-hearted presence in the clubhouse and a hard-throwing competitor on the mound.
"Tommy never has a bad day. Very infectious personality, and he can pitch," Orioles manager Buck Showalter said. "He will help the Cubs. I'm glad it's to a place that's competitive."
The trade was announced shortly after Friday's non-waiver trade deadline. Minutes earlier, Hunter joked with reporters about the rumors he would be traded, offering the shoes in his locker at a bargain price.
Soon after that, he was summoned into Showalter's office to be told the news.
"That's a tough one," Showalter said. "He makes it easier by how upbeat he was. He was picking us up in there."
Lake batted .224 with one home run and five RBIs in 21 games with the Cubs this season.
The Cubs also designated right-handed pitcher Ben Rowen for assignment.
AP Sports Writer David Ginsburg in Baltimore contributed to this report.