Lionel Loh, who was on a two-day visit, complained about the bugs, saying the problem took place as he was staying at Bangkok Cha-Da Hotel, where he checked in on March 30 with a Thai friend.
"My whole body was very itchy, especially my legs and hands, and it is very unbearable," he said of what happened in the early hours of March 31.
"I and my Thai friend went to the hotel and requested some medicated oil for the itching and was informed that the hotel did not provide such a service. We then went to the nearby 7-Eleven to buy the medicated oil," he said.
A doctor in Singapore, whom he visited on Tuesday upon his return home, said bed bugs were "likely" to be the cause of his itching. The doctor's examination was seen by the Bangkok Post.
Mr Loh lodged a complaint with the Tourism Authority of Thailand (TAT) and urged the agency to look into the case.
"Your complaint will be forwarded to the related travel provider and they should reply to you directly," the TAT replied in a e-mail.
He booked the hotel through the Agoda service.
Hotel manager Kitiphat Houngtin on Sunday said the hotel was aware of the case and dismissed the cause as bed bugs. "I can confirm that there are no bugs at the hotel," he said. All rooms and amenities were regularly checked and cleaned, he added.
The case was unprecedented at the hotel, he said.
Mr Kitiphat said he checked with staff on duty that night and was told the guest had an itch on his right hand. "The duty staffer decided not to give him medicated oil due to concerns that he could be allergic to it," he said.
The hotel has not received any querries from the TAT and Agoda but the manager said he was ready to explain what happened to the two agencies.