"He has those late-inning experiences, and he's been throwing the ball well," manager Mike Matheny said. "A big arm with experience is a great addition to our club."
The NL Central-leading Cardinals sent minor league outfielder Malik Collymore to their division rivals as part of the deal, announced hours before the non-waiver trade deadline.
St. Louis receives $1 million from Milwaukee to cover part of Broxton's $9 million salary. His contract includes a $9 million mutual option for 2016 with a $2 million buyout if declined by the club, and the Cardinals would get an additional $2 million for the Brewers by Nov. 14 if the option is not exercised.
Brewers general manager Doug Melvin said the parties discussed a deal a week ago and got it done after Milwaukee "bought down some of his contract.'"
The 31-year-old Broxton has struggled this season in Milwaukee. The right-hander is 1-2 with a 5.89 ERA, but he had not allowed a run his last seven appearances.
Last week, St. Louis acquired another right-hander, former closer Steve Cishek, from Miami, also for a minor leaguer.
"We felt confident he was trending in a real strong direction," general manager John Mozeliak said. "Putting him into our bullpen with Cishek will help the manager in terms of making sure (Seth) Maness and (Kevin) Siegrist don't get overused."
Mozeliak said the Cardinals filled their other trade deadline objective, a left-handed hitter, by acquiring first baseman-outfielder Brandon Moss from the Indians on Thursday.
The last-place Brewers dealt outfielder Gerardo Parra earlier Friday to Baltimore for minor league pitcher Zach Davies.
Broxton and Parra were acquired at last year's deadline by the Brewers, who were contending at the time. Broxton had a career-high 36 saves for the Dodgers in 2009.
Collymore was a 10th-round draft pick in 2013 and has been playing at rookie-level Johnson City.