Metcalfe will leave the Glasgow Warriors for warmer climes in a bid to claim the Heineken Cup with the strong French side.
But Castres are in a tough Pool, and will face NEC Harlequins, Munster and the Neath-Swansea Ospreys.
Metcalfe had hoped to remain with Glasgow for the forthcoming campaign but committed himself to a move to France by the time an acceptable offer was made from his current club.
New Zealand-born Metcalfe made 40 appearances for his adopted nation before retiring from the international stage but was keen to see his club career remain at the top level.
When his chances of staying with the Warriors next season looked slim, he enlisted the help of former Scotland team-mate and Irish-based agent Shaun Longstaff.
Longstaff said: "Glenn was the benchmark for all rugby players in Scotland for some time in terms of attitude and ability both on and off the field.
"France will offer him a last taste of elite level rugby before he returns to his native New Zealand on the international stage but was keen to see his club career remain at the top level."