The 19-year-old only featured five times for the Whites in the Premier League last season, and he will now be looking to gain regular first-team football in the second tier with the Lions in 2022-23.
Cresswell, who has already linked up with his new teammates in training, becomes Gary Rowett's fourth signing of the summer window following the permanent captures of Benik Afobe, George Honeyman and Zian Flemming.
Speaking to Millwall's official website on his arrival at The Den, Cresswell said: "I'm delighted to be here. I'm going to be giving my all for this club and for this moment in time I'm a Millwall player, I'll do everything I can to help this club and I'm sure I'll show you that.
"I think I'm a player who brings a lot of determination and aggression, I always give 100% and I'll try my very best. I think Millwall is a good club to learn at, it comes with a lot of pressure and expectation and I think that's what I need and I'm looking forward to getting going.
"I'm looking forward to it, I've heard what [the fans] are about and I'm looking forward to getting out there and showing what I can do."
Cresswell has spent his entire career with Leeds since joining the club's academy in 2013 and has made a total of 40 appearances at youth level, captaining the Under-23s in the last two seasons.
The teenage defender has also won six international caps for the England Under-21s team, including his debut in November last year. body check tags ::