The Republic of Ireland Under-19 stopper has been at Molineux since November 2009, joining the club on a permanent basis in January 2010 from Monaghan United in Ireland.
Impressing for the reserves and during a loan spell with AFC Telford, he is now looking to make the next step and prove his ability at League One level.
Saddlers manager Dean Smith told the club's official website: "Aaron is a talented young lad who will come here and compete for a place. We've watched him a few times and have noted his progression.
"I feel that he will come here and strengthen us in what is an important position."
McCarey has signed for Walsall for an initial month and subject to the necessary clearance being received, he will be available to make his debut for the club at Crawley Town on Saturday.