The talented second-row proved an instant hit with the Robins after joining on a short-term deal in June following a controversial exit from his hometown Wigan Warriors.
Tomkins, 31, is a dual-code international having also represented England’s rugby union side during a three-year stint with Saracens.
He bolstered Rovers’ bid for Super League survival and extending his deal was key in the eyes of head coach Tim Sheens,
Tomkins initially signed until the end of 2019 but will now be at Craven Park until the end of 2021.
“The idea is to really establish the team longer-term and not go year by year in terms of changes to the squad,” Sheens said.
“That is why we have tied down a number of our senior players to long-term deals, of which Joel is certainly one.
“Joel has become a leader within the group and is setting high standards for the boys as a senior figure, he fits perfectly with the culture we are trying to create at the club.”
Tomkins, who won the Super League title and Challenge Cup with Wigan playing more than 200 games in two spells, added: “At the end of the season I had a chat with Neil (Hudgell) and Tim where they offered me a three-year deal and I jumped at the chance.
“It really gives me an opportunity to really stick my teeth into it and I’m happy to be spending the next three years at Hull KR.
“Joining halfway through the season was quite tough with coming into a new environment where all the boys knew each other.
“But the boys and the staff were all fantastic with me, and I have thoroughly enjoyed my time here so far.
“The fans have been really welcoming as well as the whole city, I have really enjoyed my time here so far and now moving over to the city gives me an opportunity to experience a different culture.”