The 28-year-old midfielder made 11 appearances for The Lions earlier this season during a successful loan spell which saw Kenny Jackett's men lose just once. And for his part, Bolder is more than happy to be returning.
"I'm pleased to be back," he told the club's official website. "There was an offer from a Championship club but having talked it over with my family, I decided that Millwall is the club for me.
"I enjoyed my time here earlier in the season and now I am looking forward to playing my part in what hopefully will be an exciting and successful second half to the campaign."