The back will join former Scarlets and Wales team mate Leigh Halfpenny at the Crusaders, though the latter is currently sidelined with injury and has had to sit idle as the side suffered back-to-back defeats at the start of the season.
McNicholl left the Crusaders to join Scarlets in 2016 and won the first of his 10 Wales caps four years later after serving his residency period. He last played international rugby in 2022.
"I'm really looking forward to running out in front of the home crowd again, I didn't think I'd get that opportunity again so yeah, it's pretty special," McNicholl said in a Crusaders statement.
"It's also great for me and my family to be coming back home after a decent stint overseas."
He has been brought in as cover for New Zealand international Will Jordan, who is set to miss the entire Super Rugby season following shoulder surgery last month.
"Johnny's a top-notch operator with proven experience and like any good fullback, he has the potential to produce magic from anywhere on the field," Crusaders coach Rob Penney said.
