The robber, who wore a balaclava to hide his face, targeted the McGill's driver as he waited at a terminal in Brediland Road.
Police said the 28-year-old driver spotted the man approaching the bus and opened the doors, believing it was a passenger.
He was then threatened with a knife by the man who demanded cash and ran off with a small amount of notes and change.
Inspector Steven Espie, of Johnstone Community Police, said the young driver was left "badly shaken" by the ordeal.
A spokesman for the bus firm said: "McGill's is working with police to catch the individual involved.
"Incidents of this nature are extremely rare, however drivers and other customer-facing staff should be able to do their job without the threat or fear of intimidation or violence.
"The safety of our staff is paramount and we will always push for prosecution and the highest level of sentence in every case like this."
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