Terms were not disclosed, but Fitzpatrick received a two-year contract, a source told ESPN's Cameron Wolfe. A source told ESPN's Adam Schefter that the $11 million contract contains incentives that could boost the deal to between $17 million and $20 million.
Prior to Fitzpatrick's addition, the Dolphins had just two quarterbacks on their roster: Jake Rudock and Luke Falk. Neither has started an NFL game, and they have combined for five career passing attempts.
Miami is expected to continue its search for a franchise quarterback in the NFL draft -- in 2019 or 2020.
Fitzpatrick, 36, stepped in for the suspended Jameis Winston in Weeks 1-3 last season and led the Tampa Bay Buccaneers to upset wins over the New Orleans Saints and Philadelphia Eagles. He became the first quarterback in NFL history to throw for 400 yards in three consecutive games and led the NFL with 1,230 passing yards in that span.
He would step in for Winston again in 2018 when the former No. 1 overall pick was benched. Fitzpatrick played in eight games with seven starts, finishing with 17 touchdowns and 12 interceptions and completing 66.7 percent of his passes -- the highest completion percentage of his career.
Fitzpatrick's greatest issue is stringing consistent games together over long stretches of time, as he struggled with interceptions when former Bucs coach Dirk Koetter moved him into a starting role.
Fitzpatrick, who has also played for the Rams, Bengals, Bills, Titans, Texans and Jets, has thrown for 29,357 yards with 190 touchdowns and 148 interceptions in 141 games. He could become the first player to throw passes for eight NFL teams, breaking a tie with six other players, according to ESPN Stats & Information.
He also becomes the second owner of the "FitzMagic" nickname on the Dolphins roster. In September 2018, defensive back Minkah Fitzpatrick filed a trademark application with the United States Patent and Trademark Office to own the nickname in hopes of selling merchandise and apparel.
Minkah Fitzpatrick, who said he had the nickname since high school, had his trademark request denied in January.
ESPN's Jenna Laine contributed to this report.