The centre, who has 58 caps for Wales and represented his country at two World Cups in 2011 and 2015, has spent the last three seasons at the Liberty Stadium.
In that time the 30-year-old has struggled with injuries – a back problem was the biggest issue – but he is now back fit as he prepares to play his rugby in Llanelli once again.
“I have some wonderful memories playing for the Scarlets. It is my home side and I have a lot of good friends at the club,” Williams said.
“I am looking forward to working under Dwayne Peel and the coaching team in pre-season and challenging for a place in the side ahead of the start of the new campaign.
“Scarlets have some great centres and competition is going to be strong, but I’m hungry to get back out there and can’t wait to get started and pull on the Scarlets jersey again.”
After coming through the Scarlets Academy, Williams made his senior debut in a pre-season friendly against Neath in 2008 and went on to play 137 games over nine seasons, scoring 23 tries.
He was an instrumental part of the side’s stunning Guinness PRO12 title win in 2017 and scored key tries as the Scarlets reached the semi-finals of the Champions Cup a year later.
Williams has also had a fine Test career.
He scored a hat-trick of tries for Wales against Namibia at the 2011 World Cup and helped Wales to win the Six Nations Grand Slam a year later, famously crossing against England.
Scarlets general manager of rugby Jon Daniels said: “We are delighted to be welcoming Scott back to Parc y Scarlets.
“Scott has been unlucky with injuries in recent years but he is fit, in great shape and raring to go.
“He is a player who has a huge amount of experience at international level and that, along with his leadership qualities, will provide a huge value to this group.
“Scott has proven his pedigree throughout his career. He is a determined and driven character, a player the boys look up to, and we are all looking forward to seeing him in a Scarlets shirt again.”