A sheet processing device is configured to separate a non-bonding portion of a two-ply sheet in which two sheets are overlapped and bonded together at one end as a bonding portion of the two-ply sheet. The sheet processing device includes a first sheet feeder configured to feed the two-ply sheet, a second sheet feeder configured to feed an inner sheet to be inserted between the two sheets of the two-ply sheet in a state in which the non-bonding portion of the two-ply sheet is separated, and circuitry configured to cause the first sheet feeder to feed the two-ply sheet, perform a sheet separating operation on the two-ply sheet to separate the non-bonding portion of the two-ply sheet, and cause the second sheet feeder to start feeding the inner sheet before completion of the sheet separating operation on the two-ply sheet.