The 23-year-old former Western Suburbs Magpies player will join up with the Sharks from next season.
Cronulla Coach Ricky Stuart says Collis has plenty of untapped potential.
“You only have to go back two years ago and Dean had a representative experience playing for City and I still think his best football is in front of him,” Stuart told Foxsports.
Collis’ move coincides with Luke Covell‘s decision to extend his contract with the club until the end of 2010.
The news is a welcome turnaround for Cronulla fans, who have had to endure a season of off-field controversy.
In May, Chief Executive Tony Zappia was forced to step down after a string of allegations were made about his behaviour towards a female member of staff, while star halfback Brett Seymour was sacked last month after breaking the club policy on alcohol.