He made his debut against Essex in 2010 and has gone onto play a further 62 games in the Vitality Blast, with a highest score of 90 against Yorkshire in 2018.
The all-rounder is in Australia with the England squad for The Ashes.
He was absent from the summer's marquee Test series against India and also missed the Twenty20 World Cup to focus on his mental wellbeing.
In addition, he was troubled by a finger injury, suffering a fracture to his left index finger at the Indian Premier League in April, necessitating surgery. Stokes was still struggling with lingering discomfort in the area when he aborted a tentative comeback in July.
He had a second operation to remove two screws and was subsequently cleared to join the national team.
Director of cricket, Marcus North added: “Ben is one of the finest players in the world and while we may not see him as much as we would like his influence around the club remains huge.