Type species: Hydnobius matthewsi Crotch, 1874: 74.

Diagnostic description. Body large, elongate, length of pronotum + elytra = 3.1–6.4 mm; antennal club interrupted 5-segmented; mandibles elongate, broad at base and narrow apically, apices entire; labrum emarginate; clypeus strongly sclerotized; epistomal suture clearly indicated laterally, usually faintly visible medially; eyes relatively small, weakly or not protuberant; post-ocular tempora not developed; gular sutures widely separated basally and apically, convergent near middle; no suture separating gula from submentum (except in M. simulator); pronotum with sub-basal impressed line or row of punctures; mesoventrite weakly oblique, not carinate (except in M. simulator); mesocoxae narrowly separated by mesoventral and metaventral processes; outer margin of male protibia prolonged beyond base of tarsi; male protibia with elongate, sinuate apical spur; femora lack distinct teeth on posterior margins.

The following characters of Macrohydnobius separate this genus from Hydnobius: body large, length of pronotum + elytra = 3.1–6.4 mm; mandibles elongate with apices entire (bidentate in Hydnobius); lacking a suture separating gula from submentum (present in Hydnobius); pronotum with sub-basal impressed line or row of punctures; male protibia with elongate, sinuate apical spur (lacking in Hydnobius); femora without distinct teeth on posterior margins (metafemoral tooth present in Hydnobius).

Distribution and diversity. The genus is known to contain only the following Nearctic species. We have not been able to recognize that any existing Palearctic species should be placed in this new genus.

Etymology. From the Greek macros, long; refers to the elongate shape and large size of these beetles; and Hydnobius.