Miami have acquired running back Lamar Gordon from St Louis in return for a third-round pick in the 2005 NFL Draft.
The Dolphins had been trying to fill the void created by the surprise retirment of Ricky Williams.
"We're happy with the backs that we have, but this was an opportunity for us to go out and get another player that we think can come in and help us win games," said Miami general manager Rick Spielman.
Gordon was a back-up to Marshall Faulk for the Rams last season and rushed 71 times for 298 yards and one touchdown. He also had eight receptions for 59 yards.
When Faulk was injured last season, Gordon started twice and rushed for 173 yards on 40 carries before suffering an injury of his own.
He had surgery on his left ankle in training camp this year, but returned to rush for 77 yards in the Rams' pre-season finale against Oakland.
Coach Dave Wannstedt had recently named Travis Minor his starting running back, to be supplemented by Sammy Morris.
The team had been taken by surprise when Williams abruptly announced his retirement in July.
Williams had rushed for 3,225 yards and 25 touchdowns in two years in Miami.