The Cardinals sent Double-A pitcher Kyle Barraclough to Miami for the sidearming Cishek. The deal came a week before baseball's trading deadline.
Cishek is 2-6 with three saves and a 4.50 ERA this year. The 29-year-old righty had trouble earlier this season and was sent down to Double-A, but had an 0.71 ERA in 13 games since returning to Miami in mid-June.
"He's a guy that we never enjoyed facing," Cardinals manager Mike Matheny said before Friday night's game against Atlanta. "We're excited to have him and we think he can help us out right away. I've already talked to him. He's excited. He's a guy who wants to come right in and make an impact."
Cardinals general manager John Mozeliak said he had been looking to add a piece or two.
"When you look at this club and how we're put together right now, clearly there are things we can do to improve," he said.
"One of the places we felt like we could focus on was the bullpen. You look at what Steve brings and how he pitches, it just seemed like it could be the right balance for what we're trying to do," he said. "He's had a lot of success. I just feel like what he's capable of doing will really help us out."
Cishek has recently been among the most durable relievers in the majors, pitching at least 67 games in each of the past three seasons. He had 39 saves for the Marlins last year and 34 in 2013.
"Obviously, he's done a lot of good things. I'm excited to get him here," Cardinals reliver Randy Choate said. "He struggled for a while, but he worked his way out of it."
The Cardinals have the best record in the majors. Miami is fourth in the NL East.
Barraclough was a combined 3-0 with 12 saves and a 2.27 ERA in Single-A and Double-A this season. The 25-year-old righty was drafted in the seventh round by St. Louis in 2012.