The former Huddersfield, St Helens, Warrington and Wigan centre had been widely tipped to join the Black & Whites since his departure from Champions Wigan last month.
Long commented: “To be able to boast two class centres in Martin and Kirk Yeaman in our side would be great for us.
“I know when I came to Hull it gave me a new lease of life and I felt fresh and ready to take on the new challenge. I’ve spoken with Martin and think coming to Hull can do the same for him.”
The experienced centre had been training at Barrow with brother Mark to keep up his levels of fitness and is expected to join up with his new team mates shortly.