It is welcome business for the club given he has been one of their most influential players in recent seasons.
Griffin, 31, was tipped to make his England debut earlier this season until cruelly rupturing his Achilles tendon in the Challenge Cup semi-final loss to St Helens in June.
He has scored 35 tries in 110 appearances since joining from Salford Red Devils in 2017 and developed into one of Super League's finest centres.
“It was a really easy decision for me to stay here," said Griffin, who will now stay until the end of 2023.
"I love being here in Hull and at the club. Me and my family have settled down nicely in the five years we have been here and to be staying here for at least another two years is fantastic.
“All my focus is on returning to action next year and playing my part in helping the team return to the levels we know it should be at – that’s challenging for trophies.
“I’m planning on a return for Round 1 of Super League. I’ll be back in full training by December and hoping to even make a push to take part in some of the pre-season trial games.”
Hull FC head coach Brett Hodgson said: “Keeping Josh for another two years was an easy decision - you saw the strength he brings us when he hasn’t played this year, because we’ve really missed his influence.
“He’s grown into one of our leaders and is vocal amongst the players every day. He is one of the best centres in the competition and an important part of our team.
“His diligence around some of his training when he’s had his injury has gone up to another level too. He’s well on his way to recovery and will have plenty of time to get himself right for the new season.”