The Bruins will receive a fourth-round draft pick in 2011.
The deal had to be approved by the NHL, which technically is running the Coyotes following bankruptcy proceedings last fall.
"We are very excited to have brought Derek Morris back to the Coyotes organization," Coyotes general manager Don Maloney said in a statement. "Derek is a talented, versatile defenseman who will strengthen our blueline. His leadership and tenacity will be welcomed in our dressing room."
Morris went from Phoenix to the New York Rangers a year ago at the trade deadline, then signed with the Bruins in the offseason.
He has 25 points in 58 games for the Bruins this season and will be an unrestricted free agent this summer. Morris, who had a no-trade clause, said there was only one team -- the Coyotes -- for which he would waive it.
"I'm really excited for Phoenix, obviously," he said. "I think they have an amazing coach [Dave Tippett] there. A coaching staff ... You've got three of the best coaches that I feel probably are in the game and that's probably why those guys are winning."
Morris was asked if he was stunned the Bruins asked him to waive his clause.
"You never say stunned, because I think we expect things like that to happen in the game," he said. "Every day I went home, with the way the season's went, and just wondered if it had been my fault or whatever."
Information from ESPN.com NHL writer Scott Burnside and ESPNBoston.com writer Matt Kalman was used in this report.