Williams signed for the Toffees in 2016 after a number of years impressing at Swansea but now temporarily joins Stoke as they bid to bounce back from Premier League relegation.
Potters manager Gary Rowett said, as quoted by the club's 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.”
Chief executive Tony Scholes added: “Ashley’s pedigree and experience speaks for itself and I have no doubts that he will be a fantastic addition to our squad in the Championship.”
Everton have so far signed Richarlison and Lucas Digne from Watford and Barcelona respectively this summer and are likely to use Michael Keane, Phil Jagielka and Mason Holgate as their main centre back options for the upcoming 2018/19 season.