Barclay, a classic Scottish open-side flanker, has since made 146 appearances for the Warriors and has amassed 41 caps for Scotland to date, having made his debut for the national side a day after his 21st birthday. He faced Richie McCaw and the All Blacks on his first outing at Murrayfield in the Rugby World Cup.
John Barclay said: "The club has been a huge part of my life since I turned professional and I've learnt a lot from the coaches and players that I've worked with at the Warriors.
"I'd like to thank everyone associated with the club for their support over the last nine years and I'm looking forward to being involved in some big games between now and the end of the season."
Glasgow Warriors head coach, Gregor Townsend said: "John has been a fantastic player for the club and we would like to wish him all the best for the future.
"He will be with us until the end of the season and is now back to full match fitness following his recent hamstring injury. It is great to be able to call on a player of his ability and experience for what could be some of the most important games in the club's history."
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