Since dioxolane and oxathiolane nucleosides have exhibited promising antiviral and anticancer activities, it was of interest to synthesize an isosteric class of compounds, oxaselenolane nucleosides, in search of biologically interesting nucleosides. Despite their structural similarity to known 3′-heteroatom-substituted nucleosides, the synthesis of oxaselenolane nucleosides has been elusive due to the oxidative elimination characteristic of selenides. We developed a synthetic method using selenol acetic acid as a precursor, employing a one-pot reaction to avoid isolating unstable selenol acetic acid, and prepared racemic oxaselenolane pyrimidine nucleosides. The structures were confirmed by elemental analyses, 1H and 13C NMR, 2D-NOESY experiments, and single-crystal X-ray crystallography. Antiviral evaluations showed that cytosine (9a) and 5-fluorocytosine (9b) analogs exhibited potent anti-HIV activity (EC50 0.88 and 0.51 µM, respectively) and anti-HBV activity (EC50 1.2 µM for both compounds) with no toxicities up to 100 µM in PBM, CEM, and Vero cell lines. The R-isomer (10a) also exhibited moderately potent anti-HIV activity with no toxicities up to 100 µM. In summary, the first synthetic method for a novel class of oxaselenolane nucleosides was developed, leading to cytosine and 5-fluorocytosine nucleosides with potent anti-HIV and anti-HBV activities. The synthesis and biological evaluation of related purine and pyrimidine analogs, as well as the preparation of optically pure oxaselenolane nucleosides, are in progress.