The Rockies claimed Castano off waivers Tuesday and optioned him to Triple-A Albuquerque.
Castano was designated for assignment by the Mariners last week but will maintain a spot on a 40-man roster with the Rockies. The right-hander opened this season at the Triple-A level and has given up one earned run with a 4:1 K:BB across 6.1 innings.