The Diamondbacks selected Castillo's contract from Triple-A Reno on Thursday.
Castillo got a late start to the season with Reno due to an adductor injury, but he made five appearances there and allowed just one run with a 7:0 K:BB over 5.1 innings. The left-hander missed much of 2024 with a hand injury, and when he makes an appearance, it will be his first in the majors since 2023.