A sheet discriminator, which is incorporated in an image forming apparatus and an image forming system, includes a sheet loader on which a recording medium is loaded, an information detector including a light emitter to emit light to a surface of the recording medium loaded on the sheet loader and a light receiver to receive the light emitted by the light emitter and detecting information of the recording medium, a sheet distinguisher to distinguish a type of the recording medium based on the information detected by the information detector, and a light emission controller to control activation and stop of the light emitter and activate the light emitter before the information detector starts detection of the information of the recording medium.