We evaluated a one-step sandwich-format microplate enzyme immunoassay for detecting dengue virus NS1 antigen (Ag) in human serum by use of Platelia Dengue NS1 Ag kits (Bio-Rad Laboratories, Marnes La Coquette, France). We collected 299 serum samples from patients with dengue disease and 50 serum samples from patients not infected with dengue virus. For the 239 serum samples from patients with acute infections testing positive by reverse transcription-PCR and/or virus isolation for one of the four dengue virus serotypes, the sensitivity of the Platelia Dengue NS1 Ag kit was 88.7% (95% confidence interval, 84.0% to 92.4%). None of the serum samples from patients not infected with dengue virus tested positive with the Platelia Dengue NS1 Ag kit. A diagnostic strategy combining the Platelia Dengue NS1 Ag test for acute-phase sera and immunoglobulin M capture enzyme-linked immunosorbent assay for early-convalescent-phase sera increased sensitivity only from 88.7% to 91.9%. Thus, NS1 antigen detection with the Platelia Dengue NS1 Ag kit could be used for first-line testing for acute dengue virus infection in clinical diagnostic laboratories.