The 21-year-old, who can play in midfield or upfront, agreed terms with The Lions earlier today to team-up with his former boss at Eastlands Kenny Jackett once again.
"I will be joining Millwall because I see this as a great opportunity to play regular first team football for a club that clearly has the potential to progress to the Championship," Grimes told the club's official website. "You only have to look at the stadium, the set-up and the manager to see that."