The 27-year old attracted interest from several rival clubs, including the Wests Tigers, Newcastle Knights and Parramatta Eels, but has agreed to terms on a new deal to remain at South Sydney until the end of 2024.
The Bunnies have received their second major boost for the week with Cook recommitting as Wayne Bennett was unveiled as their head coach for 2019 on Tuesday.
It is understood that Bennett has been in contact with Cook in recent weeks in a bid to keep him at the club beyond next season.
The re-signing of Cook is a major win for the club with their premiership window wide open for the next few seasons.
The Kangaroos hooker has played 73 NRL games, including stints for the St George Illawarra Dragons and Canterbury Bulldogs.