The first Tomocerus species with a postantennal organ (PAO) in the adult stage is described from Vietnam. Tomocerus postantennalis sp. n. differs from the other PAO-possessing tomocerid, Tomolonus reductus Mills, 1948, mainly in the morphology of PAO, the number of ocelli, the number of chaetae in trochantero-femoral organ and several features of the furca. The new species is placed in Tomocerus because of the presence of a toothlet on the outer basal mucronal tooth and the absence of the diagnostic character states of Plutomurus Yosii, 1956 and Aphaenomurus Yosii, 1956. Besides the presence of PAO, the new species is peculiar in having six prelabral chaetae, instead of four as in other Tomocerus species. The new species is similar to Tomocerus folsomi Denis, 1929 and Tomocerus ocreatus Denis, 1948 in the type of dental spines but different from them in the body colour, the relative length of antennae to body, the number of unguis inner teeth and the number of mucronal intermediate teeth.