Barclay, who joined the Warriors straight from school as a fresh-faced 17-year-old, is now in his sixth season with the club and has become one of Scotland's most formidable flankers.
Commenting on today's news, Barclay said: "The club is definitely going in the right direction and very soon we will have an outstanding training facility in Scotstoun that will put us right up there with some of the best clubs in Europe.
"I have great friends and family in Glasgow and we have a very settled squad here at the Warriors. I feel the club can go on and win the Magners League, if not this year, then in the very near future."
John is homing in on is 100th appearance for the club having featured on 98 occasions, scoring 11 tries.
Warriors head coach Sean Lineen said: "John has been around for a while now but is still very young so I'm delighted that he's decided to secure his future with Glasgow.
"I've seen him grow into a classy international player and it says a lot about the character of the man that he wants to stay in Glasgow and win."
Meanwhile, Barclay's back-row colleague, Kelly Brown, will leave the club at the end of the season to join Guinness Premiership side Saracens.
Lineen said of the 26-year-old: "Kelly is in a rich vein of form at the moment and will be a loss to us. We had hoped to retain his services but understand his reasons for moving south.
"Even though Kelly is moving on, I know he'll give everything to help Glasgow win something this season.
"It's been a pleasure to work with him because he is a consummate professional and I wish him all the very best for the future."
Brown joined the Warriors from Border Reivers in 2007 and has gone onto make 55 appearances for the Warriors scoring five tries.
On his pending departure, Brown said: "I've thoroughly enjoyed my time in Glasgow and will hopefully leave the club following their highest ever finish in the Magners League.
"I thank Sean Lineen and the coaching staff for their support over the past two seasons and look forward with excitement to the new challenges that lie ahead with Saracens."
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