Lamont will play the rest of his season at Parc y Scarlets and expressed his thanks to his colleagues and supporters in the Scarlets region - saying it had been a difficult decision and as much a personal one as professionally with the draw of returning to his home roots.
Lamont returns to Glasgow Warriors with his brother fellow Scottish international Rory Lamont who has also re-joined the Scottish club after leaving Toulon.
On his decision Lamont said: "I am very grateful for the opportunities I've been given at the Scarlets - it's been an excellent time for me and one of the best moves of my career to join such a positive and energetic team environment and the chance to play a brand of rugby I thrive on and enjoy.
"I would like to express my sincere thanks to Nigel Davies and the coaching team at the Scarlets for all they have done to support me as a player. The opportunities I've had in the Scarlets shirt for the past three seasons have re-ignited my enjoyment and spirit for rugby and have no doubt enhanced and promoted my international career.
"The Scarlets are a very close-knit team, the set-up has strong values and it's been good to be part of a first-class professional environment at Parc y Scarlets.
"The boys are great to work alongside and the traditions and style of rugby engrained in the West Wales community is one that leaves a huge mark on you as a player.
"Out of respect for the Scarlets jersey and my fellow team-mates, the region and supporters can be sure I will be giving my all to the rest of this season's campaign - we have some big and intense games ahead of us in both the league and Heineken Cup and I look forward to the chance to show how important this club has been to my career."
Head of Rugby at the Scarlets, Nigel Davies said: "Sean has been a powerful player for us and brought experience into a squad at a key time when we were nurturing and developing a number of young backs. As one of our senior players, he has brought a lot to our environment not only in terms of the impact of his own performances on the pitch but the influence of his professionalism and commitment on a group of younger players in training.
"He has shown great respect for our values and philosophy as a rugby group and we all wish him very well back home in Glasgow and respect and support his decision to move at the end of the season."