The 29-year-old, who joined the club last January from Norwich City in a swap deal with Luciano Becchio, is returning to a club he helped win promotion from League One in 2010.
Morison had said this week that he wanted to fight for his place at Leeds, but he has been a big disappointment since his move north. He has made 16 appearances for United, scoring three goals.
The striker reported back for pre-season training at Thorp Arch yesterday, but was in London today finalising the terms of his switch to Millwall.
United manager Brian McDermott said: “I am looking to re-shape my squad for the new season and there was an offer on the table for Steve to go to a club that he has already played at.
“He’s there for a year and he will be playing games and due to the terms of the deal he can’t play against us.”