Watts was made available by Rovers boss Craig Sandercock in the last two weeks and it was the Black and Whites who have come out on top to sign the services of one of Super League’s brightest young players.
The prop joins Hull FC on a three-and-a-half-year contract which will take immediate effect.
The 21-year-old explained how excited he was when he heard of Hull FC’s interest.
He said, “Hull are a club that’s on the rise and I couldn’t quite believe it when my agent contacted me and said Hull were interested in taking me on. I just wanted to sign with them straight away.
He said, “Peter (Gentle) is really eager to build a youthful squad that can stay together for a number of years and I really wanted to be part of that.
“Hull is a club that don’t want to finish outside the top eight, but the ambition has got to be higher than that and I want to be competing for trophies here.
“There are not many teams capable of signing Gareth Ellis and the aim for us going forward has to be finishing in the top four and competing in the Grand Finals.”
Watts made his debut at Craven Park at the age of 17 where he went onto make 50 appearances for the East Hull side.