BACKGROUND: Global influenza virus circulation decreased during the COVID-19 pandemic, possibly due to widespread community mitigation measures. Cambodia eased some COVID-19 mitigation measures in June and July 2020. On 20 August a cluster of respiratory illnesses occurred among residents of a pagoda, including people who tested positive for influenza A but none who were positive for SARS-CoV-2. METHODS: A response team was deployed on 25 August 2020. People with influenza-like illness (ILI) were asked questions regarding demographics, illness, personal prevention measures, and residential arrangements. Respiratory swabs were tested for influenza and SARS-Cov-2 by real-time reverse transcription PCR, and viruses were sequenced. Sentinel surveillance data were analyzed to assess recent trends in influenza circulation in the community. RESULTS: Influenza A (H3N2) viruses were identified during sentinel surveillance in Cambodia in July 2020 prior to the reported pagoda outbreak. Among the 362 pagoda residents, 73 (20.2%) ILI cases were identified and 40 were tested, where 33/40 (82.5%) confirmed positive for influenza A (H3N2). All 40 were negative for SARS-CoV-2. Among the 73 residents with ILI, none were vaccinated against influenza, 47 (64%) clustered in 3/8 sleeping quarters, 20 (27%) reported often wearing a mask, 27 (36%) reported often washing hands, and 11 (15%) reported practicing social distancing. All viruses clustered within clade 3c2.A1 close to strains circulating in Australia in 2020. CONCLUSIONS: Circulation of influenza viruses began in the community following the relaxation of national COVID-19 mitigation measures, and prior to the outbreak in a pagoda with limited social distancing. Continued surveillance and influenza vaccination are required to limit the impact of influenza globally.