Rory Harley Davidson and another man were thrown from the Suzuki bike after it struck a stone dyke on Boig Road, near Nith Bridge, at 06:15 on Monday.
The 23-year-old, from New Cumnock, died at the scene.
The other man, whose name has not been released, remains in a critical but stable condition in Ayr Hospital. Police have appealed for witnesses.
Police have released the name of a man who died after a motorbike crash near New Cumnock in East Ayrshire.