The squad will depart for France on Tuesday morning where they will spend the week in camp ahead of their game on Friday evening.
Castres are starting the new season as Top 14 champions after defeating Heineken Cup winners Toulon at the Stade de France on June 1.
Commenting on the selected squad, Connacht head coach Pat Lam said: "I'm very pleased with the preparation and conditioning that has been done to date.
"As we head into a critical phase in our pre-season, the squad will get some valuable game-time and the opportunity to test out our new systems and structures against the French champions and in our other pre-season games against London Wasps and Newcastle Falcons.
"Bringing such a large squad to France allows us to give every player some valuable game-time and a chance to perform ahead of the new season."
CONNACHT Squad (v Castres Olympique) - Backs (17): Gavin Duffy, Shane Layden, Fionn Carr, Steve Macauley, Eoin Griffin, Danie Poolman, Conor Finn, Brian Murphy, Matt Healy, Tiernan O'Halloran, Dan Parks, Miah Nikora, Craig Ronaldson, Frank Murphy, Marcus Walsh, Kieran Marmion, Paul O'Donohoe.
Forwards (20): George Naoupu, John Muldoon, Willie Faloon, Jake Heenan, Mata Fifita, Andrew Browne, Danny Qualter, Conor Kindregan, Aly Muldowney, Michael Kearney, Nathan White, Rodney Ah You, Ronan Loughney, Jason Harris-Wright, Sean Henry, James Rael, Finlay Bealham, Brett Wilkinson, JP Cooney, Michael Swift.
Players on stand-by - Peter Reilly, Tadhg Leader
Unavailable/Injured/College exams - Denis Buckley, Craig Clarke, David Nolan, David Heffernan, Eoin McKeon, Aaron Conneely, Jack Carty, David McSharry, Kyle Tonetti, Darragh Leader, James So'oialo, Robbie Henshaw.