The structural, tautomeric, hydrogen-bonding, stacking, and electronic properties of a seleno-derivative of thymine (T), denoted here as 4SeT and created by replacing O4 in T with Se, are investigated by means of ab initio computational techniques. The structural properties of T and 4SeT are very similar, and the geometrical differences are mainly-limited to the adjacent environment of the C-Se bond. The canonical "keto" form is the most stable, tautomer, in the gas phase and in aqueous solution, for both T and 4SeT. It is argued that the competition between two opposite trends, i.e., a decrease in the base-pairing ability and an increase of the stacking interaction upon incorporation of 4SeT into a duplex, likely explains the similar experimental melting points of a seleno-derivative duplex (Se-DNA) and its native counterpart. Interestingly, the underlying electronic structure shows that replacement of O4 with Se promotes a reduction in the HOMO-LUMO gap and an increase in interplane coupling, which suggests that Se-DNA could be potentially useful for nanodevice applications. This finding is further supported by the fact that transfer integrals between 4SeT center dot center dot center dot A stacked base pairs are larger than those determined for similarly stacked natural T center dot center dot center dot A pairs.