He will be called upon to help set up for closer Francisco Rodriguez.
"I like the pieces we have in our bullpen but we're talking about a guy that's a pretty special guy,'' manager Ron Roenicke said. "He's got great stuff, he competes well, he's a good teammate and I think it puts another piece there where I can pitch guys exactly how I want to.''
Broxton, 30, is 4-2 with 7 saves and a 1.86 ERA in 48 1/3 innings this season. He has 118 career saves.
He is owed about $1.2 million for the rest of 2014, based on his $7 million salary, and $9 million for 2015.
"Part of the thinking was his contract. We had to recognize we're taking on this contract for next year, too,'' general manager Doug Melvin said. "You try to get somebody who can help. There's a lot of big money guys today, whether they're relievers or [position] players, whatever. We've done a pretty good job with bullpens without having to spend a lot of money.''
Broxton is scheduled to join the team Monday in Chicago and is eligible for the postseason roster because the move was made prior to Sept. 1. His addition from a division rival will be a big boost to a beat-up, taxed bullpen -- and an acquisition Milwaukee had been looking to make.
Milwaukee entered Sunday's games with a one-game lead in the National League Central over the St. Louis Cardinals.
The Associated Press contributed to this report.