A total of 1,141 rotavirus-negative stool specimens collected from diarrheic children in 4 distinct regions under sentinel surveillance in Thailand between 2006 and 2008 were examined by reverse-transcription (RT)-PCR for norovirus (NoV) and sapovirus (SaV). Three hundred 3 specimens (26.6%) were positive for NoV, with 34 and 269 belonging to genogroup I (GI) and genogroup II (GII), respectively. Twelve specimens (1.1%) were positive for SaV. Mixed infections were found in 5 specimens: 3 samples indicated the presence of both NoV GI and GII, and 2 samples indicated the presence of both NoV GII and SaV. Analysis of the monthly distribution of NoV and SaV revealed that NoV GII was clustered between September and February, while NoV GI was detected mainly in June and July; SaV was found in May, June, and July. In addition, 3 outbreaks of acute gastroenteritis at 2 junior high schools in Phichit and Bangkok, and at a university in Phitsanulok, Thailand in 2006 were found to have been caused by NoV infection. Sequence analysis of NoVs from sporadic cases and outbreaks showed them to be genotypes GII.4 and GII.6.