Nainlin Formation (new) is proposed to unify two Middle Cambrian successions of “red beds” that crop out in the eastern Mackenzie Mountains, Northwest Territories. Strata east of Mackenzie Arch previously were treated as a proximal facies of the Saline River Formation whereas equivalent strata further west were included as a basal member within the Franklin Mountain Formation. Both successions are characterized by red-weathering siliciclastic facies that preserve evidence for deposition in shallow to marginal marine settings, commonly under restricted conditions. Whereas typical Saline River Formation is dominated by mudrocks and evaporites, Nainlin Formation is rich in quartzose sandstone, lacks evaporites, and locally preserves conglomerate, notably in sections studied along the east flank of Mackenzie Arch. Although Nainlin Formation locally contains dolostone, its siliciclastic-dominated character distinguishes it from the dolostone-dominated Franklin Mountain Formation. Nainlin Formation lies with angular unconformity upon Neoproterozoic and earlier Cambrian formations and is overlain conformably, generally gradationally, by Franklin Mountain Formation. It is in part age-equivalent to Hess River and Rockslide Formations, which lie to the west and south, but has not been mapped in continuity with them. The western limit of Nainlin Formation is a zero edge in the footwall of the Plateau Fault in NTS 106A; the formation extends from thence into the easternmost Mackenzie Mountains, where it passes laterally into Saline River Formation. Published age controls from underlying and overlying units constrain Nainlin Formation as no older than Delameran (early Middle Cambrian) and not younger than late Marjuman (late Middle Cambrian). Nainlin Formation generally is less than 100 m thick, although locally it can exceed 300 m in thickness. It is preserved in three outcrop belts separated by elongate paleohighs that apparently were “corrugations” on the broad crest of Mackenzie Arch. The eastern high is here dubbed “Sheep Lick high”. Further west, paleohighs can be mapped in the footwall and hanging-wall of Plateau Fault; these are dubbed “Deadend high” and “Sayunei high”, respectively. The paleohighs and intervening outcrop belts are oriented NW-SE, as is the facies transition from Nainlin Formation into Saline River Formation.