Pneumonia is a leading cause of morbidity and mortality among infants and young children. The most common causes of pneumonia in children are respiratory viruses. In Thailand, the epidemiology of the viruses causing community-acquired pneumonia (CAP) among children is poorly defined. In this cross sectional study we used nasopharyngeal samples collected from hospitalized children diagnosed with severe CAP in accordance with WHO criteria between June 2013 and May 2014 to determine the causes of infection. The samples were analyzed for respiratory syncytial virus (RSV), parainfluenza viruses (PIV) types 1,2 and 3, adenovirus, rhinovirus, influenza viruses types A and B and coronavirus by polymerase chain reaction (PCR) and reverse transcriptase-polymerase chain reaction (RT-PCR). Of 102 cases of severe CAP, samples were obtained in 91 cases and 48 (52.7%) were positive for respiratory viruses. The most common viruses were RSV (n = 22; 45.8%), rhinovirus (n = 11; 22.9%) and adenovirus (n = 9; 18.7%). Patients were aged 1 month to 4 years 5 months, with a median age of 1 year 1 month. Thirty-seven (77.1%) were male. Asthma was the most common co-morbidity affecting 5 (10.4%) of the 48 cases with an identified virus. The peak prevalence occurred during October (n = 17). All patients required oxygen therapy and 17 (35.4%) required mechanical ventilation. The median length of hospitalization was 11 days. Preterm infants had a significantly higher rate of RSV infection than other respiratory viruses (8 of 21; 38% vs 3 of 27; 11.1%) (p = 0.02). Viruses were most commonly associated with severe CAP among children aged less than 1 year. The peak prevalence occurred during the rainy season. Our findings suggest that young and preterm infants with CAP should be monitored closely due to their high risk for developing serious complications.