John Lowrie Morrison OBE, known as "Jolomo", who studied at the school for five years, will give financial help to the Mackintosh building fire fund.
News of his support comes the day after Hollywood star Brad Pitt and Scots actor Peter Capaldi gave their backing to a £20 million fundraising campaign to restore the structure.
The city and the arts world were rocked on May 23 when flames engulfed the Grade A-listed building, leading to the loss of around a tenth of the structure and 30% of its contents.
Jolomo, who studied at Glasgow School of Art from 1967-72, said he was devastated when he heard about the fire.
He said: "I heard the news on radio in my studio. I had to stop painting, I was close to tears.
"The Mack made me who I am, it has a big place in my heart. I'm here, a successful artist painting the Scottish landscape, because of all I got from the Mackintosh building and the people in it - and the teachers who are there now are continuing to do that for folks like myself."
He called on other former students to rally round and support the appeal. "They will restore the building, and it's up to the alumni to help fund it.
"It has done so much for so many artists over the years, it's time to give something back. People who benefited should be helping out now, even a few pounds adds up."
The painter will give a percentage of the proceeds from Jolomo in the Hebrides, which opens on Saturday at MacGregor Fine Art in Glasgow, and several other exhibitions this year to the fund.
Restoration of the fire-damaged building could cost £20-£35 million and take up to four years.
Professor Tom Inns, director of Glasgow School of Art, said: "We have been overwhelmed by the offers of support that we have received since the fire last month.
"We are very grateful for this wonderfully generous offer from John Lowrie Morrison which will help us to ensure that, like him, future generations of students can study and make work in the inspirational Mackintosh building."
After graduating from GSA, Jolomo worked for 25 years in art education before leaving to paint full-time in 1997.
Celebrities such as Sting, Madonna and Rick Stein are among those who have bought his work, which features distinctive colourful landscapes of Scotland.