We investigated the prevalence, seasonal dependence, and genetic diversity of noroviruses (NoVs) in the Tamagawa River, which runs through a densely populated region in Tokyo, Japan, from April 2003 to March 2004. A total of 60 water samples were collected from five sites from upstream to downstream along the river, and 500 mL of which was concentrated using the "cation-coated filter method". Of the 60 samples tested, genogroup I (GI) and GII NoVs were detected from 28 (47%) and 18 (30%) samples, respectively. GIV NoV was successfully detected from 2 (3%) samples with a newly developed seminested RT-PCR assay specific for GIV. The occurrence of NoVs in the river was significantly higher in winter/spring than in summer/autumn and also in mid- to downstream rather than upstream. A total of 176 different NoV strains were identified from river water samples based on the phylogenetic analysis of partial capsid gene sequences. GI, GII, and GIV strains were clustered into 7 (GI/1, 2, 4, 5, 8, 9, and 11), 8 (GII/2, 3, 4, 5, 6, 11, 12, and 16), and 1 (GIV/1) genotypes, respectively. The results suggest that genetically diverse NoV strains are circulating between human populations and aquatic environments.