A fixing device includes an endless belt formed into a loop, a heater to heat the endless belt, a nip formation assembly disposed inside the loop, and a pressure rotator to press against the nip formation assembly to form a fixing nip between the endless belt and the pressure rotator. The nip formation assembly includes a pressure pad and a supplementary thermal conductor to conduct heat from the heater in an axial direction of the endless belt. The supplementary thermal conductor has an edge portion dimensioned to distance the supplementary thermal conductor from the endless belt at an end portion of a face over which the endless belt slides in a longitudinal direction of the supplementary thermal conductor. A distance between the edge portion of the supplementary thermal conductor and the endless belt increases toward an end portion of the endless belt in the axial direction thereof.