The 25-year-old former Celtic player makes the move to Toronto with the aim of bolstering a back line that has conceded 48 goals in the MLS so far in 2012, second only to 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 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 his debut 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.
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