Barclay said: "It was an emotional and memorable evening on Friday night in front of a packed home crowd and it was great to get my 150th appearance and bid farewell to supporters on the back of an important win for Glasgow in our stadium.
"I am now really looking forward to moving to the Scarlets next season; having seen the ambition they play with, the style, rugby philosophy and tradition of such a great rugby region, I know that it will be a great move for me.
"I've been lucky to be with Glasgow for nine years and helped take Glasgow to develop into what is now seen as a very competitive side.
"Having been with them all my professional career, to move somewhere new, it had to be to the right club with an environment and philosophy I could relate to and to a team that I could make a difference within.
"It's going to be an exciting opportunity for me to experience a new environment in West Wales in the first move of my club career. They have an impressive and ambitious coaching group and it will be good to be part of a team that has so much talent, potential and energy with a mix of key world-class players along with world class playing and training facilities.
"I know that Sean Lamont really enjoyed his time at Parc y Scarlets and he's been giving me a few pointers ahead of my move and I look forward to being part of a highly-respected Welsh region with a great heritage and loyal support base."
There is plenty left to play for in the 2012/13 season with Glasgow safely through whilst the Scarlets are also challenging for a play-off position.
Scarlets Head Coach Simon Easterby said: "We are very pleased to have signed John; he will add real value to our group both in terms of his own exceptional ability and experience but also will be a great mentor to a number of our younger back-rowers coming through the system.
"We made clear strides with strong front five recruitments last season to ensure we are strengthening the forwards unit to give the right platform to deliver to our half backs and backs and John will play a key role in the continuing development of that area of our game.
"We want to have strength and depth in our pack and the back row is a key area in our game moving forward.
"John is a dynamic player who will offer another added dimension to the existing talent we have in our back row. He has shown that he is really at the top of his game since coming back from injury. At just 26, he brings world-class experience yet still has his best rugby years ahead of him."