The study reports on an epidemiological investigation of an influenza A(H3N2) outbreak which occurred in a manufacturing company in Thailand during September 2015. The workplace consisted of three buildings. Employees in building 1 did not wear protective equipment and masks while those in buildings 2 and 3 wore C-level suit for protection from chemicals inhalation. The disease spread and involved 216 (8.4%) cases from a total of 2,585 employees. Nine out of 18 throat swab samples were found to have influenza A(H3N2) virus. Influenza illness mostly occurred in building 1, with attack rate of 22%. The investigation revealed that the first case possibly contacted the disease from a family member and spread it among employees through direct contact with clinically active cases, and sharing of hand towels in the company toilets. The study emphasized practical control measures, particularly in health education and strong policy regulations in the workplace. It enforced all employees in building 1 to wear masks which lead to the successful control of the outbreak within 10 days without using oseltamivir post-exposure prophylaxis. The event-based surveillance system should be implemented in every workplace for outbreak detection as well as for rapid response.