The 26-year-old is set to bring an end to his Warriors career, which began at the age of 17 after joining as an apprentice in 2004.
He made his debut in November of that year as a substitute against the Borders, and has gone on to make 146 appearances for the club.
Barclay, who has 41 caps, 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 head coach Gregor Townsend added: "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."
- Sports & Recreation