The striker becomes manager Steve Lomas' second signing of the summer as he plans for the new season.
Morison spent two years at The Den between 2009 and 2011, scoring 40 goals in 95 games, and moved onto Norwich before January's switch to Elland Road.
He told the Millwall website: "I'm pleased to be back here.
"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."