The deal finalised today, a one-year extension with an option for a further year, allows the classy back-row forward to maintain his link with a club with whom he has been synonymous since joining as an apprentice in the summer of 2004.
Barclay has long been regarded as one of the most gifted openside operators in the world game, his displays in the RaboDirect PRO12 and Heineken Cup winning him widespread recognition from an early age.
He is set to have a major say in the Warriors' RaboDirect PRO12 play-off push, having returned from national service to state a case for selection in Friday's crunch game against Cardiff Blues at Firhill (kick-off 7.35pm).
This season the 25-year-old has been able to demonstrate positional versatility, switching to both blindside flanker and No.8 when required and managing to bring to bear his usual eminent mix of composure, good hands and mastery of the breakdown.
The back row, a consistent area of excellence in the Warriors team this year, is set to become even more of a strong suit next season, with Tonga's Rugby World Cup No.8 Viliami Ma'afu and All Black Angus Macdonald recruited to supplement the already impressive current options.
Barclay, who has amassed some 126 Glasgow appearances and 12 tries, told www.glasgowwarriors.org: "I'm delighted to be re-signing for the Warriors at a time when the club is really going places. I've been part of Glasgow for eight years now, and the atmosphere and ambition around the club is the best I've ever experienced.
"It's always a big call to decide where your future lies, particularly when you've been with one club for a significant period. I've taken some time to think through the decision, and I'm convinced that it's the right one. Glasgow Warriors are going places and I want to contribute to that process.
"I'll be giving my all for Sean Lineen and the rest of the coaches as we try to reach the RaboDirect PRO12 play-offs, and will be doing likewise when Gregor [Townsend], whom I know well from Scotland duty, takes over as head coach next season."
Lineen commented: "It's been great to see John develop from an 18-year-old with all the right raw materials into a proper Warrior who, on his day, is right up there with the best in the business.
"At 25, he should be just starting to come into his prime, and I'm very pleased he's decided that Glasgow is the best place to continue his development and achieve what he wants to in the game."
Managing director Nathan Bombrys said: "The quality of display that John Barclay has produced consistently over the years is one of the reasons why Glasgow Warriors have reached a stage where we're challenging for honours.
"We see John as a player who can contribute significantly to us maintaining and improving that status, and it's very positive news for the club to have him secured on a new contract.
"The competition for places in our back row has been fierce all season, and that's only going to intensify next year with the arrival of Viliami Ma'afu and Angus Macdonald. That sort of internal rivalry is great for the club, because it drives up standards across the board."
Barclay is the 17th frontline Warriors player to sign a contract extension in recent months.
In addition to starring for Glasgow, Barclay has won 37 caps for Scotland, playing a part in both the 2007 and 2011 Rugby World Cups and being named Man of the Match in the milestone wins over Argentina in Mar del Plata and South Africa at Murrayfield, both in 2010.
Tickets for Glasgow Warriors v Cardiff Blues are available via www.glasgowwarriors.org, by calling 0844 800 3490 or in person at the Gordon Street and Sauchiehall Street branches of Greaves Sports.
Adult tickets purchased in advance cost just £15 - a £5 saving on the match-night rates.
On Friday we'll be raising funds for Yorkhill Children's Foundation, one of our official charity partners. The Warriors match jerseys will sport the Foundation's logo, and we'll be raffling the number 4 shirt at the game.
Raffle tickets available pre-match and at half-time from the Greaves Sports outlet in the concourse of the Jackie Husband Stand and the Warriors Supporters Bar, accessed from Firhill Road.
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland