A sheet separation device includes a rotator, a nipping member to nip the two-ply sheet with the rotator, a first conveyor to convey the two-ply sheet in a first direction toward the rotator and a second direction reverse thereto, a winding member to wind the two-ply sheet around the rotator, a second conveyor to convey a sheet medium in the second direction, a separation claw to move in a width direction; a sheet guide to guide the separated two sheets to different passages; and control circuitry. The first conveyor conveys the two-ply sheet to the rotator with a bonded portion at an upstream end. The control circuitry inserts the separation claw into a space between the two sheets, adjusts a projection amount of the two-ply sheet from a nip of the first conveyor in the second direction, and inserts the sheet medium in two-ply sheet adjusted in the projection amount.