The 51-year-old driver was dropping off a man in Irvine when the passenger threatened him with a knife and demanded money.
The victim handed over a small amount of cash before the suspect made off in an unknown direction.
Police said the robbery took place on Warrix Avenue last Wednesday at about 18:40.
The driver had picked the suspect up at Irvine train station a short time earlier.
Police said the taxi driver was uninjured but badly shaken by his ordeal.
The suspect was described as being a slim white man in his late teens with a local accent. He was wearing a light-coloured hooded top.
Det Con Alex Greig said: "The victim was simply doing his job when he was threatened at knifepoint for fares he had collected.
"Although uninjured, this was a traumatic incident for the driver and we are doing all that we can to trace the man responsible.
"Our inquiries are continuing and we continue to analyse CCTV in the area. We are seeking assistance from the public to help trace the suspect."
Police are hunting for a man who robbed a taxi driver at knifepoint in North Ayrshire.