Allan trained with his new club on Monday after South Sydney granted him permission to explore his options elsewhere given Latrell Mitchell blocked his path to the Rabbitohs' No.1 jumper.
A breakout 2020 campaign that ended with a Maroons debut at fullback debut saw Allan's value and standing rise accordingly, with Bulldogs coach Trent Barrett chuffed at adding another quality recruit to his ongoing rebuild.
"We have worked hard in the off-season to sign the right players in terms of their ability and attitude and we believe that Corey will be a great addition for our club," Barrett said.
"Everyone has seen what Corey can do over the past two seasons and we believe that he has the potential to be an outstanding player and someone that adds both quality and versatility to our backline."
The three-year deal ties Allan to the Bulldogs until the end of 2023 and sees him join the likes of Nick Cotric, Kyle Flanagan and Jack Hetherington as off-season recruits.
The 22-year-old's signing has him in line for the fullback role that has been rotated between Dallin Watene-Zelezniak, Will Hopoate and Nick Meaney in recent seasons.
While more than happy to accommodate Allan in 2021, Rabbitohs head of football Mark Ellison acknowledged that taking Canterbury's offer made sense for the rising star in South Sydney's release statement.
"Corey has done a great job for our club over the past two seasons and he goes to his new club knowing he has the respect of everyone at Souths," Ellison said.
"His preferred position, and his best position, is fullback and the opportunity to play in that position on a regular basis is not there with us.
"He feels he will have a better opportunity to play fullback at his new club, on an extended and improved deal than that which he had with us.
"We felt it would be unfair to stand in the way of Corey progressing his career somewhere else, he will always be a Rabbitoh, and we wish him and his partner Teigan all the best for the future."