Foster, 30, released a letter on his decision to retire via Uninterrupted on Monday.
"There comes a time in every athlete's career when their ambition and their body are no longer on the same page. I've reached that point," Foster wrote.
Foster had been a four-time Pro Bowler for the Houston Texans, but suffered a torn?Achilles tendon in October 2015 and was released during the offseason in March.
He signed?with Miami to a one-year, $1.5 million dollar deal one week before training camp.
He finishes his career with 6,527 rushing yards and 54 touchdowns, and left Houston as the franchise record holder in rushing yards.?
"This game has been everything to me ... my therapy, my joy, my solace and my enemy. I've learned to love every facet of this game, from the peak of accomplishment to the gutter of criticism, " Foster wrote.
Foster added: "I am walking away in peace. I know it is not commonplace to do it midseason but my body just can't take the punishment this game asks for any longer."
The retirement is effective immediately.
The Dolphins met with him in the spring to get an update on his Achilles rehab and kept him on the short list before signing him in July.
Foster won the starting job during the preseason, but his tenure as the starter lasted just two games. Foster injured his groin and a hamstring in Miami's Week 2 loss to the New England Patriots and missed the next three games.
By the time Foster returned in Week 6, he didn't look the same, and Jay Ajayi made significant strides as the starter. Ajayi became just the fourth player in the Super Bowl era to post back-to-back 200-yard games against the Pittsburgh Steelers and Buffalo Bills.
Barring injury, Foster likely would have been a backup the rest of the season had he not retired. Foster also made headlines this season as one of three Dolphins players, along with receiver Kenny Stills and safety Michael Thomas, to take a knee during the national anthem before every game.
ESPN's James Walker and The Associated Press contributed to this report.?