Johnstone spent a loan spell at Glanford Park last season, and played against Walsall in a 2-2 draw at the Saddlers, but failed to impress in a struggling side.
He becomes the seventh keeper the Saddlers have included in their matchday squad this season.
"I know a lot about the lad and have had some very positive reports on him," said Walsall boss Dean Smith.
"He has already had a couple of previous loan spells at this level, so he knows what it's all about.
"Having watched him in action recently, I'm delighted to be able to bring him on board until the end of the season."
