Thomas received a two-year deal, a source told ESPN.
Thomas, 28, was a captain each of the past two years for the Dolphins.
Thomas was also one of several Miami players to protest social injustices for minorities by kneeling during the national anthem. The protests have been a topic of discussion this past week after notable free-agent safety Eric Reid said his involvement in them is the reason he still has not been signed.
Thomas began his career with the San Francisco 49ers as an undrafted free agent out of Stanford. The versatile defensive back -- who can play both safety spots and nickel cornerback -- has 25 career starts in his five seasons with the Dolphins.
Thomas also could improve the Giants' special teams coverage. His 55 special teams tackles over the past four seasons tied him for the league lead. He also led the Dolphins with 11 special teams tackles in 2017 and tied for the league lead with 19 special teams tackles in 2016.
Thomas comes to the Giants as a familiar face to defensive backs coach Lou Anarumo, who was his position coach in Miami. Anarumo joined the Giants this offseason after six seasons with the Dolphins.
The Giants were looking for a veteran safety this offseason to add to their defensive backfield, where Landon Collins and Darian Thompson were the starters. Thompson had his ups and downs in his first year as a starter, even though he played the most snaps out of anybody on the defense.