We did a seroepidemiological study to determine the circulating dengue virus serotypes and the extent to which the Singapore population has been exposed to multiple dengue virus serotypes, using the plaque reduction neutralization assay (PRNT). Of 164 enrolled subjects aged between 18–30 years, 49 subjects (29·8%) were PRNT positive for at least one dengue serotype. The seroprevalence was 39 (23·8%) for dengue virus serotype 1, 37 (22·6%) for type 2, 43 (26·2%) for type 3, and 30 (18·3%) for type 4. Of the 49 subjects with PRNT-positive dengue virus results, 28 (57·1%) were positive to all four virus serotypes, seven (14·3%) to three serotypes, two (4%) to two serotypes, and 12 (24·5%) to a single serotype. All four dengue virus serotypes circulate in Singapore, and a substantial proportion of the adult population in Singapore had exposure to more than one dengue virus serotype. In spite of multiple circulating types, the rate of dengue haemorrhagic fever is low in Singapore.