Shane Lowry leaves Aston Villa in a short-term loan move to the New Den until January 2, the club have confirmed.
The 22-year-old will be reunited with Villains team mate Nathan Baker, who is also on loan at Millwall after not featuring this season for the Midlands club.
Lions boss Kenny Jackett has now lured two Villa defenders to the New Den in the space of two days.
Lowry is yet to have appeared in the Premier League, but has featured in the Europa League and experienced promotion from League One while on loan at Leeds United.
The Australian defender has also had successful loan spells at Plymouth Argyle (2009) and Sheffield United (2011) despite not being able to prevent the Blades being relegated to League One last term.
Villa boss Alex Mcleish extended Lowry's contract when he put pen to paper with a new two-year contract in the summer.
This keeps the defender at Villa Park until 2013, but the boss allowed him to leave for Millwall in order to gain valuable experience.