Connacht coach Michael Bradley sees the announcement of these matches as crucial to his team's preparation for the coming season's Magners League campaign.
"Castres and Harlequins are two quality sides and when you consider we'll be playing South Africa the week before Castres, we should find ourselves in a position to hit the ground running once the Magners League kicks off," the former Ireland scrum-half said.
"We've always tried to get pre-season games against stiff opposition and this season is likely to be the toughest we've ever had. It's going to be a great test for us and also great for Connacht supporters to see these sides and we're looking forward to the games already."
These matches will be covered by Connacht Rugby Season Tickets. There are a limited number of tickets still available for the South Africa match.