Bus captain Saw Cheong Seng did not pay much attention to a bag that a passenger on SBS Transit service 63 handed to him on Friday morning.
Taking a quick look, he saw two stacks of paper inside.
When Mr Saw, 59, returned to the Eunos bus interchange, he was stunned to find that what he had thought to be betting slips were actually $100 and $50 notes.
The two stacks, each several centimetres thick, together with some change, came up to $39,602.10.
The senior bus captain said the passenger, a man who looked to be in his 40s, had handed him the bag at around 8.40am at the bus stop opposite Outram Park MRT station.
The man had told him that another passenger had left it behind.
An SBS Transit spokesman said the money was handed over to the police and that a police report had been made.
Mr Saw, who has been with SBS Transit for the past 13 years, said he wanted to make sure every cent was returned to its rightful owner.
He said he had found money on the bus before, but the largest amount was about $100.
"I would not be able to sleep tonight if I had taken this sum of money that does not belong to me," he told The Sunday Times.
"I would feel troubled if my passengers are not able to get back their lost items, especially their money."
• Additional reporting by Derek Wong