Williams, nicknamed ‘Sanjay’, played amateur rugby while working as a scaffolder in the Port Talbot steelworks.
He started 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 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 – who paid tribute to coaches including Warren Gatland, Neil Jenkins and Nigel Davies – signed off with the message: “Lastly 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.”
