The Scotland international has signed a two-and-a-half-year deal with the option of a further 12 months after being granted a free transfer.
There was some doubt whether the Baggies had completed the deal before the deadline, with Fletcher arriving back at their training base from a medical after the 11pm deadline but the club later confirmed a deal for the midfielder had been completed in time.
Fletcher was seen leaving United’s Carrington training ground late on transfer deadline day and filmed arriving at West Brom soon afterwards to finalise the transfer.
The 31-year-old had been set to join West Ham United last week but the deal broke down after the Hammers tried to land him on loan, with Louis van Gaal stating publicly that was “not the agreement”.
Fletcher is a product of United’s academy and leaves Old Trafford after more than 300 appearances for the club, and 24 goals from midfield.