The 34-year-old arrived from Swansea City in the summer of 2016 for £12 million on a three-year contract.
Williams featured in 36 league games in his first season and in 24 in 2017-18, making a total of 73 appearances before leaving for Stoke City on loan last summer.
The Wales international featured in 37 games for the Potters, but that wasn’t enough to earn him a new deal at Everton, with manager Marco Silva fully convinced he doesn’t fit into his plans.