Dengue virus (DENV) infects 50-100 million people worldwide per year, causing severe public health problems. DENV RNA-dependent RNA polymerase, an attractive target for drug development, catalyzes de novo replication of the viral genome in three phases: initiation, transition, and elongation. The aim of this work was to characterize the mechanism of nucleotide addition catalyzed by the polymerase domain of DENV serotype 2 during elongation using transient kinetic methods. We measured the kinetics of formation of the elongation complex containing the polymerase and a double-stranded RNA by preincubation experiments. The elongation complex assembly is slow, following a one-step binding mechanism with an association rate of 0.0016 ± 0.0001 μM(-1) s(-1) and a dissociation rate of 0.00020 ± 0.00005 s(-1) at 37 °C. The elongation complex assembly is 6 times slower at 30 °C and requires Mg(2+) during preincubation. The assembled elongation complex incorporates a correct nucleotide, GTP, to the primer with a K(d) of 275 ± 52 μM and k(pol) of 18 ± 1 s(-1). The fidelity of the polymerase is 1/34,000, 1/59,000, 1/135,000 for misincorporation of UTP, ATP, and CTP opposite CMP in the template, respectively. The fidelity of DENV polymerase is comparable with HIV reverse transcriptase and the poliovirus polymerase. This work reports the first description of presteady-state kinetics and fidelity for an RNA-dependent RNA polymerase from the Flaviviridae family.