On June 4, 2016, the Prey Veng Provincial Health Department reported a total of 107 patients with influenza-like illness (ILI) from Chakhlanh village to the Cambodian Ministry of Health. To confirm the outbreak and evaluate its clinical and epidemiological characteristics, the investigation team visited the village and reviewed the case-based surveillance (CBS) data on severe respiratory infection (SRI) and patients' records in health facilities. The team interviewed all households in the village from May 1 to June 5, 2016 and obtained the following data: age, medical history, date of onset, treatment, symptoms, and history of contact with patients or dead poultry. Nasal swab samples were collected from suspected ILI cases to test for influenza virus by RT-PCR. The investigation detected 498 suspected ILI cases, including 288 females. Although the incidence of suspected ILI cases who visited health centers was 63.0 per 1,000 persons per month, the attack rate was 27.1 per 100 population. The major age group was 5-14 years followed by 0-4 years. Major symptoms were cough, fever, runny nose, and headache. Six of seven nasal swab samples were positive for influenza A/H1N1 pdm09 virus. Most children with flu symptoms had contact with previous cases. This study showed that the ILI outbreak might be caused by seasonal influenza A/H1N1 pdm09 spread from person to person. Poor living conditions and poor hygiene and sanitation practices were environmental factors that caused the outbreak. As the CBS system was unable to identify this epidemic, it needs to be improved.