The 25-year-old Ireland international joins Toronto FC after playing for Celtic, Reading, Ipswich Town, and Leeds.
The 25-year-old former Celtic player looks to bolster a TFC back line that has given up 48 goals in MLS play so far in 2012, second only to the expansion Montreal Impact.
"Darren addresses our need for an experienced, no nonsense central defender," Toronto head coach Paul Mariner said in a club statement. "Darren has great leadership abilities and his experience, both for club and country, will be a welcomed addition to our club and for our young players.
"We are so pleased to bring him to Toronto."
O'Dea started his professional career in 2006 with Celtic, and he also has extensive experience in the English Championship thanks to loan spells with Reading, Ipswich Town, and Leeds United. He most recently played for Leeds on a season-long loan for the 2011-12 season.
Internationally, the defender has represented the Republic of Ireland 14 times since debuting for his country in 2009, and he was included in the Irish squad for the Euro 2012 tournament.
Toronto FC's next game is Saturday at Chicago Fire, but O'Dea will not be available for selection.