A sheet processing apparatus includes a pair of first aligning members movable in a sheet conveyance direction and a sheet width direction, a pair of second aligning members to align lateral sides of a bundle of sheets, a stapler movable in the sheet width direction to staple a trailing end portion of the bundle, a driving unit to move the first aligning members, and a controller. The first aligning members include a stack portion to contact and support a trailing edge of the bundle. When the stapler staples the bundle at two positions symmetrically relative to a center position in the sheet width direction, the controller selects either first positions inside the two stapled positions or second positions outside the two stapled positions in accordance with multiple stapling-related variables, and the first aligning members are set at the selected positions to align the trailing edge of the bundle.