Off contract for over a year after leaving Ipswich Town, Williams has signed a 12-month deal with the Tigers with the option for another year.
"It feels good to be back in the team environment and back on the pitch," he told the Hull City club website. "Everyone has been very welcoming. It's been a good few weeks but I'm just happy I'm here now and can focus on the football.
"I had a couple of very honest meetings with the manager. We know it's going to be a process to get fully back up to speed, having that time out. It had been 20 months since I played or was involved with a team.
"I appreciated the manager's honesty. Now, it's down to me to work hard and get as many games as I can."
Williams added, "It was very difficult (taking a break from football) – mentally and physically. It's been a long road back, but probably something I needed. Now, I'm back here a different person," he said.
"I've grown up a lot and I'm ready to just focus on the football. I've missed it. I'm hungry, I'm fiery. I know what quality I can bring to the team. As soon as I get back to that, I'm sure I'll be an asset to the team."