Tetramorium pacificum is a medium-sized dark colored ant that is most often encountered on vegetation. This species has a monomorphic worker caste with 12-segmented antennae, three-segmented antennal club, antennal scrobes, short antennal scapes that do not surpass the posterior margin of the head, a gradually sloped mesosoma, and strong propodeal spines. Like all myrmicines, T. pacificum has two waist segments and a gaster armed with a stinger. Tetramorium pacificum has established populations outside of its native range, but it is not believed to cause significant damage to ecological or agricultural systems. The species is most often encountered on vegetation in disturbed or edge forest habitat.
Diagnosis of worker among Antkey species. Worker caste monomorphic. Head shape roughly subrectangular. Antenna 12-segmented. Antennal club 3-segmented. Antennal scapes not conspicuously short; easily extended beyond eye level; do not extend beyond posterior margin of head. Antennal scrobe present. Antennal insertion surrounded by a raised sharp-edged ridge. Posterolateral corners of head unarmed, without spines. Eyes medium to large (greater than 5 facets); distinctly less than half head length. Frontal lobes do not obscure face outline between mandible and eye; relatively far apart so that the posteromedian portion of the clypeus, where it projects between the frontal lobes, is much broader than one of the lobes. Anterior margin of clypeus notched. Mandibles triangular. Cephalic dorsum with short lateral rugae intersecting longer longitudinal rugae. Mesosoma with erect hairs. Pronotal spines absent. Propodeum armed with long robust spines. Slope of mesosoma gradual. Waist 2-segmented. Petiole with a wave-shaped node; pedunculate; lacking large subpetiolar process postpetiole attached to lower surface of gaster. Postpetiole not swollen; in dorsal view not distinctly broader than long or distinctly wider than petiole. Erect hairs moderately distributed, long and thin. Color uniformly dark brown to black.
Tetramorium pacificum is distinguished from its fellow congeners that are introduced in the United States by the following combination of characters: (1) petiolar node wave-shaped (versus evenly rounded for T. lanuginosum and T. tonganum; versus square-shaped for T. bicarinatum, T. nr. caespitum, T. caldarium, T. simillimum and T. tsushimae); (2) erect hairs long and thin (versus short and thick for T. caldarium and T. simillimum); (3) cephalic dorsum with short lateral rugae intersecting longer longitudinal rugae (versus primarily subparallel, non-intersecting longitudinal rugae for T. nr. caespitum and T. tsushimae); and (4) propodeal spines long and robust (versus small triangular teeth in T. nr. caespitum, T. tsushimae, T. caldarium and T. simillimum); (5) color uniformly brownish yellow to orange (versus dark brown to black for T. pacificum). The bicolored red and black appearance separate it from all but some specimens of T. caldarium and T. simillimum. The species looks most similar to P. insolens, but is distinguished by its darker brown to black (versus yellowish brown to orange) color.