Stoke have signed defender Williams on a season-long loan from Everton.
Boss Gary Rowett told Stoke's official website: "We have two very good senior central defenders in Ryan Shawcross and Bruno Martins Indi, but I felt it was important we brought in another experienced player to add to the competition for places.
"Having spoken to Ashley he feels like he has a point to prove and has a real hunger. He knows many of our players, knows what we're all about as a club and will fit straight in to our culture.
"He has gone for big money in the past and had a fantastic a career so far and experience in that position can be key because it can help you read the game."