Castillo was batting .163 in 24 games with two home runs, two doubles and five RBIs. He was the Cubs' starting catcher in 2013-14, batting .274 in '13, but his offense dropped last year when he hit .235.
Yahoo! Sports first reported the deal on Tuesday. Castillo is expected to back up Mike Zunino, who is batting .179 in 35 games.
The Cubs wanted to keep Castillo, 28, as insurance in case Montero, who turns 32 on July 9, or Ross, 38, was injured. A good defensive catcher in terms of throwing out runners and pitch blocking, Castillo came up through the Cubs' system.