The 29-year-old, who played for the Lions between 2009 and 2011, has been allowed to leave Elland Road by Leeds boss Brian McDermott.
The Welsh international has admitted that he is happy to be back with the South London club.
"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 moved to Leeds in January from Norwich City.