The 26-year-old, who stands at 6ft 5in and weighs 108kg, began his career on the wing before moving to the back row this year.
Okunbor has scored 15 tries in 43 NRL games since his debut in 2019, including 17 appearances for the Bulldogs in the ongoing campaign.
“I’m super excited," he said.
"It’s always been a bit of a dream of mine to play Super League so to be coming to play for a great club like Hull FC is really exciting for me.
“Hull FC is one of the biggest clubs when it comes down to rugby league so it’s a super exciting opportunity for me.”
Okunbor becomes Hull's second confirmed addition for 2024 following the signing of Herman Ese'ese, while Franklin Pele is also set to join Tony Smith's side from the NRL.
“We’re really happy to get ourselves a player who is of a good age and at a strong position in his career," said Smith on the Okunbor deal.
“Having spoken to Jayden, I can understand his determination to progress as a rugby league player and he wants to establish himself as a formidable back-rower.
“I think the attributes Jayden has and the style of play we have here make him a good fit for us. We’ve had some good discussions about how we can use him in the pack and I’m looking forward to welcoming him on board.
“He has a big frame with good speed and a good offload game so I think he has all of the attributes to develop well here.”