Travis frontman Fran Healy joked that drinking Irn Bru had turned his hair orange, as the band played a secret set at TRNSMT.
The band formed in 1990 and spent 10 years living in Berlin, as well as another eight in Los Angeles.
Some of the members have known each other since they were 17 through Glasgow Art School.
Healy, 50, sported bright orange hair and told the crowd: “I drank too much Irn Bru.”
The band agreed to the performance in a VIP area just two days ago, and released their new album on Friday, titled L.A Times.
Some people in the audience were tearful as they watched the band play hits including Flowers In The Window, and Sing.
Healy said they played at T in the Park – the predecessor of TRNSMT – on 10 occasions and it was “possible” they might play the Glasgow Green festival next year.
Bassist Dougie Payne, 51, said: “People are coming to shows who have grown up with these records from the 90s and early 2000s.
“These songs have soundtracked people’s lives.”
The group agreed that “friendship” had kept them together as a band, and Healy described them as “sensible”.
Payne said: “The thing with us is we were all friends individually, then it was four of us in the band. We have known each other as friends since we were 17.”
Healy said: “We are personalities that don’t clash, there hasn’t been anything to split us up. We are sensible.”
Guitarist Andy Dunlop said: “The secret to Travis is friendship.”
Some of the musicians said they would watch Liam Gallagher’s performance but others were heading home for an early night.
Healy said: “We played T in the Park 10 times. We would like to play TRNSMT. It’s a possibility.”
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