Stingless bees (Apinae: Corbiculata: Meliponini) are biologically and culturally important pollinators within the tropical and subtropical areas of the world. However, limited information is available for the majority of the species. Biological and systematic data are presented for a new species of Trigonisca Moure, from the arid region of La Guajira, Colombia. The genus is part of the distinctive Trigonisca genus group, noteworthy for its position as the earliest diverging extant lineage of neotropical stingless bees. We briefly diagnose the genus group and provide a key to the genera and subgenera of the Trigonisca genus group, along with the description of Exochotrigona Engel, new subgenus. We also outline the species occurring in Colombia and present a description for Trigonisca (Trigonisca) mepecheu Engel and Gonzalez, new species, including accounts of all three castes. A single, poorly preserved egg is noteworthy because of its extremely small size. Its chorion is extensively covered by a surface pattern of elevated geometric figures, as seems to be characteristic of the Meliponini. The robust mature larva, though remarkably small, exhibits extensive spiculation of dorsal body surfaces and most body segments with small, paired dorsolateral tubercles. In addition, the labral apex exhibits an apical patch of recently discovered multipronged spicules intermixed with various sensory sensilla. These morphological features of immature stages, where known, are similar to those previously reported for other Meliponini. We document the internal architecture of nests of T. mepecheu, which we found in trunks of Stenocereus griseus (Haw.) Buxb. (Cactaceae) and more commonly in Libidibia coriaria (Jacq.) Schltdl. (Fabaceae), along with nests of the only other stingless bee from La Guajira, Melipona favosa (Fabricius). Nests were also found in the sides of manufactured structures. The indigenous Wayúu harvest stingless-bee honey and have specific names in Wayuunaiki for the two species occurring in the region, although there is apparently an oral tradition in which the honey of T. mepecheu causes blindness. Trigonisca (Trigonisca) ameliae Penney from Colombian copal is a new junior synonym of T. (T.) schulthessi (Friese).