Loxandrus semperfidelis, new species (Carabidae: Loxandrini), is described from its type locality, Orellana Province, Ecuador, and distributional range extending to Peru. This large and robustly–built South American species is distinguished from all other loxandrines by the unequal widths of the elytral intervals, in which odd–numbered intervals are as much as twice as wide as even–numbered ones. Males are distinctive in having a ventral abdominal process, or “keel”, which is not known from any other species of Carabidae. Both adult and first instar larval characteristics are described.