Ellis, 32, arrived at the KC Stadium from Wests Tigers ahead of last season, and took over as club skipper, despite missing the first couple of months through injury.
New coach Lee Radford said: “Gareth’s leadership qualities have been outstanding since he arrived at the club and he’s just had just first full pre-season in about 10 years.
“Physically, he’s in great shape and he’s one of the most professional players I’ve come across during my time in the game.
“He has natural leadership qualities which have been on show throughout pre-season. Having him in from day one has benefitted him personally and also the group around him.”
He has previously played in Super League for Leeds and Wakefield.
Meanwhile, Hull have confirmed their games against Widnes and Wakefield have been reversed due to a full re-construction of the KC Stadium pitch in the summer.
Widnes will now visit the KC Stadium on Sunday March 23, with the reverse fixture now taking place on Sunday June 15, while Wakefield will visit on May 4, before the reverse fixture on June 29.
It means Radford’s side will play five consecutive away games between the home game with Wigan on May 9, and the visit of Warrington on July 4.