Williams, nicknamed ‘Sanjay’, played amateur rugby while working as a scaffolder in the Port Talbot steelworks.
He began his professional career with Scarlets – who he represented across two spells – and went on to have stints with Saracens, Cardiff, Japanese side Kubota Spears and finally Newcastle Red Bulls.
In a video posted on social media, the Swansea-born player said: “I’ve been a professional for 15 years, that takes its toll. I think it’s the right time for me to call it a day.
“I always thought I would go out on my own terms. Having knee trouble for the past few years has taken its toll.”
Williams also paid tribute to coaches including Warren Gatland, Neil Jenkins and Nigel Davies before he signed off with the message: “I’d like to thank all of the fans out there who supported me on the way.
“My journey’s ended but a new chapter is about to begin.”
