The 21-year-old will be in the matchday squad as the Saddlers face Preston if the necessary clearance is received in time.
Darlow joined Forest as a 16-year-old and has made two first-team appearances.
Manager Dean Smith told Walsall's official website: "Karl is a goalkeeper that we have watched a number of times recently.
"He is very highly-rated at Forest, so I'm delighted to be able to bring him here for a month. He will certainly strengthen us and add to the group."