A new fossil soldier beetle, Cacomorphocerus endecacerus sp. nov. (Coleoptera: Cantharidae), is described from Baltic amber. Aside from its size (7.3 mm), a unique feature of the new species is the 11-segmented antennae with the second to eighth antennomeres saucer-shaped or irregular and the final 3 antennomeres elongate. All previous species in this genus possess 12 antennomeres. Updates of the description of the genus and tribe are provided.