Hesperonychus is known from one partial pelvic girdle, holotype specimen UALVP 48778, collected by Dr. Elizabeth Nicholls in Dinosaur Provincial Park in 1982. The fossil remained undescribed, however, until Nick Longrich and Phil Currie published on it in 2009. A number of very small toe bones, including "sickle claws", in the collection of the Royal Tyrrell Museum may also belong to Hesperonychus. Despite their small size, the pubic bones were fused, a characteristic of adult dinosaurs, indicating that the specimen does not represent a juvenile of a known species.
Though known from only partial remains, Longrich and Currie estimated its total length at under one meter and weight at about 1.9 kilograms, making it the smallest known carnivorous dinosaur from North America. The alvarezsaurid Albertonykus was smaller, though likely an insectivore rather than a carnivore