The defensive lineman was released by the Patriots on Tuesday, the team announced.
Ian Rapoport of the Boston Herald originally reported the news.
Haynesworth was involved in a bit of a shouting match on the Patriots' sideline Sunday with defensive line coach Pepper Johnson. Defensive captain Jerod Mayo also got involved, as did head coach Bill Belichick. On Monday, Belichick said he keeps all conversations private, while adding that Haynesworth's lack of playing time in the second half was based solely on the rotation of the linemen.
The Patriots sent a fifth-round draft pick to Washington in exchange for Haynesworth this past summer.
Haynesworth was making just $1.5 million this season but was due nearly $7 million next season.
The 2008 Defensive Player of the Year with Tennessee, Haynesworth signed a seven-year, $100 million contract with the Redskins that offseason. He was a major disappointment in Washington, missing 12 games in two seasons and never coming close to the form he showed with the Titans.
His eight-game tenure in New England wasn't much better, with the 6-foot-6, 350-pound Haynesworth registering just three tackles and zero sacks in six games. He was inactive twice with a back injury.
The Patriots announced they signed wide receiver Tiquan Underwood, who was briefly on the Patriots this preseason.