Gallen will make history in 2019 when he takes the field for the 19th season in a row.
The 37-year-old has played 326 games for the Sharks since making his debut in 2001, a record at the club.
Gallen could reach 350 games with an injury-free 2019.
Gallen contemplated retiring at the end of the 2018 season, but after playing 22 games and having the most averaged metres gained by any Cronulla forward (138m), the 37-year-old decided to have one last crack at winning a second premiership.
Gallen has said previously he intended to play-on in 2019 to mentor the Sharks talented youngsters and strive to win a second premiership.
“I can say that 2019 will be my last season, I now have an end date and with the talent we have here at the Sharks I am confident this year and next can be successful for myself and the Cronulla club,” Gallen told the Sharks website in August.
“If I play 15 games or 25 games next year, while winning is the main aim, part of my role will also be to bring through and to help develop the next crop of talented young players at our club, as I’ve tried to do in recent years and as I’ll continue to do.”