Christian, 32, took four wickets and scored 65 runs at strike rate of 127.45 in last summer's competition.
He initially moved to Notts to replace Darren Sammy and extended his stay to include seven One-Day Cup games, taking nine wickets and scoring 251 runs.
"He got better and better over the course of his time with us," said director of cricket Mick Newell.
"We'll use him in a different role in Twenty20 cricket, higher up the order where he can make more of a batting impact."
Christian, who is playing for the Hobart Hurricanes in Australia's Big Bash competition, played the last of his 34 limited-overs internationals in April 2014.
Playing for the Hurricanes in December, he hit a remarkable 117-metre six onto the roof of the Gabba against his former side Brisbane Heat.
Australia all-rounder Dan Christian will rejoin Nottinghamshire as their overseas player for the T20 Blast.