The Royals, who also signed Stephen Quinn from the Tigers earlier this week, have given the Republic of Ireland international a three-year deal.
Manager Steve Clarke said of the 29-year-old: “Even before Alex Pearce chose to leave the club on a free transfer I had decided that another experienced central defender was needed for this season.
“I knew that Paul’s contract with Hull City was due to expire and was always monitoring the situation.
“When we met up earlier in the summer for a chat I knew that Paul would be a good signing for Reading FC and I’m pleased that we managed to get the deal completed.
“As well as his obvious talents as an experienced defender who is aggressive both in the air and on the ground, I felt that he was a good character to bring into our squad.
“Paul has gained good experience at many clubs and, like Stephen Quinn, was an important part of a promotion-winning team. He has a winning mentality and it will be good for our two young central defenders, Michael Hector and Jake Cooper, to train and play alongside Paul.”