To make room for Gaffney on the roster, the Dolphins released receiver Legedu Naanee, who fumbled after his first reception of the season last Sunday, in the fourth quarter of Miami's overtime loss to Arizona.
Gaffney, 31, has 443 receptions for 5,622 yards and 24 touchdowns in 10 NFL seasons.
"It seemed like a good fit, and they wanted me," Gaffney told reporters. "That's why I’m here. I'm looking forward to coming down here and fitting in."
He was the Redskins' leading receiver in 2011, making 68 catches for 947 yards and five touchdowns, setting career highs in receptions and yardage and matching a career high in TD catches. He started 15 games last season.
The Dolphins have been looking for a dependable No. 3 receiver behind starters Davone Bess and Brian Hartline. Veteran receiver Chad Johnson was released before the season after his arrest on a domestic violence charge.
Information from The Associated Press was used in this report.