The 23-year-old will link up with friend and former team mate Scott Dureau who has signed a two-year deal.
The childhood friends played together at Newcastle in 2004, a side which also featured Origin debutant Kade Snowden and Jarrod Mullen.
Farrar will join Catalans on a one-year deal before returning to Manly for season 2012 and 2013.
The Manly side was resigned to losing Farrar for a year after salary cap restrictions meant they could not fit the young back in their side.
Farrar’s manager George Mimis declined to comment on the deal.
“Ben’s personal life is private so I will not be commenting on what he is doing,” said Mimis.
The former Cowboy will be coached by Roosters assistant Trent Robinson.
On Wednesday 26-year-old Roosters prop Lopini Paea announced he will also join the Super League side as will Warriors hooker Ian Henderson.