To provide a sheet processing apparatus to stably align sheets having a long length, and an image forming apparatus.

When an rear-side alignment part 206 moves, a side end arranged downstream the center of gravity of the sheet S2 in a sheet conveyance direction is pressed by a second pressing member 212 provided in the rear-side alignment part 206 and pressing a large-size sheet, and a side end arranged upstream the center of gravity of the sheet S2 in the sheet conveyance direction is pressed by an intermediate stacking part side alignment part 208 that moves interlocked with the rear-side alignment part 206. Notched parts 206e and 207e are formed at corner parts in the lateral center on the downstream in the sheet conveyance direction of the rear-side alignment part 206 and a front side alignment part 207, so that the lateral center portion downstream in the sheet conveyance direction of the sheet S2 is bent downward. The second pressing member 212 projects toward the lateral center from the intermediate stacking part side alignment part 208, so that the second pressing member 212 abuts on the side end corresponding to the portion bent downward of the sheet S2.