Hull have fought off competition from virtually every Super League club to sign Ellis, who has previously played for Leeds and Wakefield.
Ellis said: “I was lucky to have a few clubs interested, but I’ve been listening to Hull and to the ambition they have got and the prospects they’re trying to put in place for the future and the bottom line for me was that it was a great time to be involved with Hull FC.
“I said originally that I would maybe like to return to one of my old clubs or my hometown club in Castleford but when I looked at it and thought about it more, I realised I still really wanted to win things and achieve things and I really believe I can do that at Hull FC.”
Ellis has won 35 caps for England and Great Britain and has forged a reputation as one of the best back-rowers in the world during his time in the NRL, winning the Wests Player of the Year three times.
He worked at Wests with Hull coach Peter Gentle, a former assistant to Tim Sheens at the Tigers.
Gentle said: “If we’re looking to take this club to another level, we need the addition of one or two class players to bring out the best of the talented squad we currently have in place and we have definitely achieved that with the signing of Gareth.
“Despite stiff competition, we are pleased he has decided to take on the challenge at Hull FC having given us the chance to present our ambitious plans for the future.”
Ellis made his Super League debut for Wakefield back in 1999 and went on to make 114 appearances for the Wildcats, before moving on to Leeds.
He was a virtual ever present in four years with the Rhinos, picking up a Super League Grand Final winners ring in both 2007 and 2008.