Using an anti-dengue immunoglobulin G (IgG) indirect enzyme-linked immunosorbent assay, seroprevalence was determined among 783 adult blood donors in the French Caribbean islands of Guadeloupe and Martinique in 2011. Overall, 93.5% [91.5; 95.1] samples were positive for dengue antibodies, 90.7% (350 of 386) in Martinique and 96.2% (382 of 397) in Guadeloupe. Only 30% of these adults recalled having had dengue disease before. Serotype-specific neutralization assays applied to a subset of IgG-positive samples indicated that a majority (77 of 96; 80%) reacted to the four serotypes. These seroprevalence findings are the first reported for Guadeloupe and Martinique and are consistent with the dengue epidemiology in these territories.