Darlow, 22, enjoyed eight appearances between the sticks for the Saddlers earlier this season before being recalled by his parent club.
And Walsall have wasted little time now the January transfer window is open in making another move for his services.
Boss Dean Smith said: "Karl did really well during his first spell with the club. It was unfortunate when Nottingham Forest had to recall him shortly after he had signed an extension to his deal here.
"We are grateful to Forest for allowing him to come back here to continue his footballing development and he will be a good acquisition to our squad."
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