The accident - involving the man and a Stagecoach vehicle - took place on Union Street close to its junction with Holburn Street.
The emergency services were called to the scene at about 21:50 on Monday.
The 59-year-old man was taken to Aberdeen Royal Infirmary, where he was said to be in a "critical" condition. Police are appealing for witnesses.
A pedestrian has suffered "life-threatening" injuries after being struck by a bus in Aberdeen.