Millwall have signed midfielder Duncan Watmore from fellow Championship side Middlesbrough for an undisclosed fee.
The 28-year-old has scored five goals in 23 appearances this season and was Boro's leading goalscorer in 2020-21.
He started his career at Manchester United's academy before joining non-league Altrincham FC aged 16.
Watmore joined Sunderland in 2013, with 87 appearances and eight goals in all competitions during a spell disrupted by several injuries.
Promotion-chasing Millwall are eighth in the Championship table and have won three of their past five games.
They host Sunderland on Saturday.