Morison, 29, returns to the Den having scored 40 goals in 95 appearances between 2009 and 2011, helping the Lions earn promotion from League One via the play-offs in 2009/10.
"I'm pleased to be back here," he told Millwall's official website. "If anybody has any doubts about me all I'd say is that I aim to score plenty of goals for Millwall and help this club to greater success.
"I've been looking after myself in the summer; I'm fit and raring to go."
Morison has scored three times in 16 outings for Leeds since joining from Norwich in January.
Leeds manager Brian McDermott told United's official website: "I'm looking to reshape 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."
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