A sheet processing apparatus includes: a folding processing unit that folds a sheet by reversely rotating a second conveying member in a condition in which the sheet is held by a first and the second conveying members; a calculating unit that calculates an amount of deflection of the sheet held by the first and second conveying members from timings at which the sheet is detected by first and second detecting units disposed upstream of the first conveying member and downstream of the second conveying member and a distance between disposed positions of the first and second detecting units; and a control unit that sets, from the calculated amount of deflection of the sheet, an amount of conveyance for the first conveying member in a direction opposite to a sheet conveying direction in a condition in which the sheet is held by the first and second conveying members.