Derry played in QPR's Capital One Cup win over Exeter on Tuesday and will be in contention for a place in the squad for Millwall's trip to face Ipswich on Saturday.
The 35-year-old, who played 18 times for the R's in the Premier League last season, has made over 600 appearances in a career spent with Notts County, Sheffield United, Portsmouth, Crystal Palace, Nottingham Forest and Leeds United.
QPR boss Harry Redknapp is in the process of reducing his wage bill at Loftus Road as Derry joins Loic Remy, Adel Taarabt and Park Ji-Sung in leaving the club on loan deals.
Derry admitted earlier this week that he would be interested in going out on loan in order to play more regularly, having spent much of last season on the substitutes' bench.
Millwall have also re-signed goalkeeper Andy Marshall on a short-term deal. The 38-year-old, who was a free agent after leaving Aston Villa, played for the Lions from 2004 to 2006.