The Let Her Go singer returned to his busking roots to raise awareness of the work of The Big Issue.
The singer said: "I don't need to play on the street with my guitar case out to try and live any more, so it's the least I can do to try and help Big Issue vendors across the country."
Hundreds of people gathered to listen to him.
Fans sang along to some of his biggest hits, such as Holes and Feather On The Clyde, which was written about Glasgow.
He also performed songs from his new album Whispers, which is out June 9.
The singer said the last time he busked in the city only 14 people gave him money.
Alex Carid, 23, said: "I was shopping in the city centre with my friends and once we found out he was there we just had to watch. He was fantastic. Loved every minute of it."
David Graves, 24, said: "His songs are so catchy, people were loving it. It's so great that he's doing all this."