A Singaporean taxi driver has been heralded as a hero after he returned $SG1.1 million ($US900,000) in cash to a vacationing Thai couple who left the money in his cab.

Sia Ka Tian, 70, has told the Straits Times he was shocked to find the money in a black paper bag on the back seat on Monday after he dropped the couple off at a shopping centre.

"When I saw the money, I thought, trouble is here. I was sure there was at least $200,000 in the bag," he said.

When he brought the money to transport company ComfortDelGro's lost-and-found office, his stunned colleagues counted $1.1 million in $1,000 bills.

"The money is unimportant to me," he said.

"It doesn't belong to me, so how can I use it?"

The Thai couple reported the loss to the transport company and Mr Sia was waiting for them when they arrived to claim the money.

The report did not say what the couple were doing with such a large sum.

The driver received an undisclosed cash reward from the grateful couple, whose names have been withheld.

Spokeswoman Tammy Tan says the company also plans to give him an award for good service.

"Finding $1 million in cash is not an everyday affair and in fact, we wonder how many people would have possibly been tempted," she said.

"We are immensely proud of him and are glad that the passengers recovered their money."

It was the most valuable item returned by a cabbie working for the company.

In 2009, another taxi driver returned five kilograms of gold bars worth $SG377,000.

AFP