Notts announced they had secured Cairns' services, subject to approval by the England and Wales Cricket Board (ECB), for their first match of the tournament against Yorkshire in Leeds on Saturday morning.
He qualifies as a local player under Kolpak regulations due to his South African wife, Carin, holding a Dutch passport.
Cairns, 37, had the first of his three stints with Notts as an 18-year-old in 1988 and bid an emotional farewell to test cricket at Trent Bridge in 2004.
"Nottinghamshire have been part of my life for 20 years and I truly thought that I had played my last game for the club so I am pleased to have the chance to make a return," Cairns said.
"I am a Nottinghamshire member, I have always kept an eye on the results and there is no other club that I would have rather joined.
"It was a late decision and the registration could come down to the wire but I'm fit to play and I can't wait to get started.
"The Twenty20 format suits my style because I like to entertain and I love winning cricket matches."
Cairns ended his international career, which included 62 tests and 215 one-day internationals, in early 2006.
He more recently played in the rebel Indian Cricket League (ICL) and captained the MCC team against New Zealand in their tour opener in April.
It means two former New Zealand test players at Notts for the Twenty20, with Andre Adams recently signed after qualifying as a Kolpak player through his St Vincent (West Indies) passport.
