Cabbie Wimon Tripat, 62, handed over the money to David Gorge Corinson, 59, at Suvarnabhumi Airport Police Station late on Friday evening.
Corinson hired Wimon from the airport to take him to Mor Chit Bus Station on Friday afternoon.
After Wimon was contacted by police to ask him if Corinson had left the money in the taxi, the driver checked the vehicle and found the cash in an envelop under the front passenger seat.
Corinson, who works in Australia, was carrying the money to give to his family in Thailand.
He thanked Wimon and gave him an unspecified amount of money as a reward.