The 26-year-old midfielder has played 86 times for Palace since joining from Ipswich in 2010, but has found first-team opportunities at Selhurst Park limited so far this season.
Garvan featured regularly under Ian Holloway during his time as Palace boss last season but fell out with the current Millwall boss when he was left out of Palace's 25-man Premier League squad.
But it appears the two have settled their differences and Garvan will be eligible to make his debut at home against Brighton on Saturday.
"Millwall are in a bit of a difficult situation but hopefully I can help play my part in turning the results around," the Irish midfielder told millwallfc.co.uk
"As a professional player you can't beat that feeling of playing on a Saturday and picking up three points, and that's what I'm here to achieve.
"I've played at The Den with Crystal Palace so I know how big a factor our home fans can be. We've got Brighton on Saturday and it's important we start on the front foot.
"We have got seven games in March alone so they will come thick and fast. I've been playing regularly for Palace's reserves so am fully fit and keen to get going, and I'll be giving it absolutely everything I've got between now and the end of the season.
"I've spoken to a few of the lads here already and they've told me about how good the club is. Coming to a new club is always a bit of a nervous time but I'm genuinely excited about being here and want to help keep Millwall safe."