Koroisau, who is fresh off his premiership-winning season at the Panthers, has become the fifth NRL player to settle his future with a rival club for 2023 this off-season.
The 29-year-old, who re-joined the Panthers in 2020, is the second player recruited by the joint venture for the 2023 season after Parramatta forward Isaiah Papali'i agreed to terms last week.
Koroisau's pending Wests Tigers move will be for two seasons with a mutual option for a third.