Garvan, 26, has penned a temporary deal at The Den, where he will be re-united with former Palace manager Ian Holloway.
The Irishman has not played in red and blue since the 2-1 Capital One Cup defeat to Bristol City in August last year, when he scored a stunning volley at Ashton Gate.
That was the most recent of his 11 Palace goals, four of which came in last season's promotion-winning Championship campaign.
Garvan will now be involved in a battle at the other end of that division, as he bids to help Millwall avoid the drop to League One.
The Lions currently sit just one point and one place outside the relegation zone, having played more games than all of the three teams below them.
Garvan could make his debut for the club on Saturday, when they host Brighton and Hove Albion.