Koroisau will play with the Panthers in 2022 before joins the Tigers, the deal has a mutual option for the third season in 2025.
The 29-year-old made his NRL debut for the Rabbitohs in 2014 and won the grand final that year. He won his second premiership this year.
The hooker has played 145 NRL games across eight seasons with the Rabbitohs, Panthers and Sea Eagles.
Koroisau has played 13 Tests for Fiji and made his State of Origin debut for New South Wales this year.
