The Lions moved for the midfielder after seeing Shaun Derry accept the manager's role at Notts County recently.
"We wanted some competition in midfield," assistant manager Mick Harford told the official Millwall website.
"Guy is big, strong and athletic, and that's exactly the type of player we've been looking for."
Moussi is the second Forest player to join a rival Championship club in recent weeks after Dexter Blackstock's switch to Leeds United.