Davies has made over 100 appearances for Rovers since joining the club in 2016.
He started his career at neighbors Castleford Tigers and has a wealth of Championship experience.
Davies has been a constant figure in Ryan Carr's side this term, making 15 appearances and scoring three tries.
"I'm thrilled," said Davies on signing his new deal.
"I've had a good successful few years here.
"I'm really happy to continue, my family enjoy coming to the games. It's a good life balance and that's why I am happy to stay."
Rovers CEO Davide Longo added: "We are delighted that John has signed for a further two seasons, he’s a quality player with a vast amount of experience.
"It was imperative that we secured his services as he is one of the best back-rowers in this division and a key member of the squad."