The deal is for two years and is worth the veteran minimum plus incentives, according to a source.
The deal reunites Blount with the team for whom he rushed for nearly 800 yards in 2013 before signing a two-year, $3.85 million deal with the Steelers in March.
The 247-pound Blount joins a Patriots running back group that includes overnight sensation Jonas Gray (who rushed for 201 yards against the Indianapolis Colts on Sunday night), Brandon Bolden, pass-catching specialist Shane Vereen and rookie James White.
"I wasn't necessarily surprised," Gray said Thursday of the move to acquire Blount. "In this business, not a whole lot surprises me, especially given where I've come from.
"I just prepare the same way -- it doesn't matter who we sign or who we bring here. I believe in Bill [Belichick] and whatever he's doing."
While Blount could help the Patriots this year, there is also an aspect of looking to the future, as running backs Stevan Ridley (on injured reserve with a torn ACL), Vereen and Bolden are all scheduled for free agency after the season.
Blount practiced with the Patriots on Thursday, and Gray said he made an instant connection with his new teammate.
"The guys here have nothing but good things to say about him," Gray said. "... I'm excited, man. I think it's going to be something good for us. For a team, what we want to do long-term, when these cold-weather games start coming up, I think it's going to help."
Blount showed his own enthusiasm in a tweet Thursday afternoon.
- LeGarrette Blount (@LG_Blount) November 20, 2014
The Steelers waived Blount less than 24 hours after he left the field before the end of the 27-24 win over the Titans because the fifth-year veteran was upset he did not receive any carries in the game. He went unclaimed on waivers.
Last season with the Patriots, Blount rushed for 772 yards on 153 carries, an average of five yards per attempt, and scored seven touchdowns. He rushed for 189 yards against the Bills in the final week of the regular season and followed that up with 166 yards on the ground and four touchdowns against the Colts in the playoffs.
He rushed for 266 yards and two touchdowns this season but carried the ball only five times in the Steelers' past two games and did not gain a yard.
To make room on the roster for Blount, the Patriots cut defensive lineman Casey Walker. Walker would be eligible to return to the practice squad if he clears waivers.