Objective: On 27 June 2023, the Thailand Department of Disease Control was notified of an incident of carbon monoxide poisoning related to a Thai-style hotpot restaurant. An outbreak investigation was performed to describe the incident, confirm its cause and sources of exposure, and provide preventive measures. Methods: The restaurant owner, restaurant guests and waiting staff were interviewed, and the medical records of hospitalized cases were reviewed. In an environmental survey, air quality parameters were measured, including temperature, relative humidity, carbon dioxide and carbon monoxide. Additionally, a simulation of the incident was conducted, and data were reviewed from previous poisoning incidents in Thailand. Results: There were 11 cases, all of whom were guests who dined in the same private dining room. The median age of cases was 28 years (range 2–62 years). Three cases were hospitalized and received hyperbaric oxygen therapy. The air changes in the dining rooms were below the recommended level. The incomplete combustion of charcoal in a poorly ventilated room led to carbon monoxide build-up, which caused the incident. The simulation experiment showed a high concentration of carbon monoxide (mean 183.16 ± 55.15 parts per million), above the standard level. Ten similar poisoning incidents occurred between 2019 and June 2023, totalling 23 cases and 2 deaths; none occurred in a restaurant. Discussion: Charcoal use in poorly ventilated areas poses a health risk, especially for children. The use of charcoal stoves for hotpot cooking indoors is prohibited. Public health policy should mandate regular restaurant inspections to ensure compliance with occupational and environmental health standards.