A sheet processing apparatus in an embodiment includes a processing unit, a tray, alignment members, and a supporting member. The processing unit processes regular size sheets. The regular size sheets can be placed on a first surface of the tray. The alignment members are capable of coming into contact with end portions in a first direction, in which a line of intersection of the first surface and a horizontal plane extends, further in an upward direction than a center of gravity of a largest regular size sheet present in an initial position where the regular size sheet entering the tray is placed first. The supporting member supports a lower end portion of the largest regular size sheet and extends from inner sides to outer sides of end portions of the largest regular size sheet in the first direction.

A stacking unit includes a processing tray, an alignment section, a transport section, and a moving section. The processing tray has a placement area in which one or more paper sheets are placed. The alignment section aligns leading edges of the paper sheets. The transport section transports the paper sheets to the alignment section. The moving section moves the paper sheets in X directions. The alignment section includes: a first alignment surface having a first friction coefficient; and a second alignment surface having a second friction coefficient. The second alignment surface is positioned upstream of the first alignment surface in a +A direction and closer to a center of the placement area in the X directions than the first alignment surface is.

A sheet processing apparatus in an embodiment includes a processing unit, a tray, alignment members, and a supporting member. The processing unit processes regular size sheets. The regular size sheets can be placed on a first surface of the tray. The alignment members are capable of coming into contact with end portions in a first direction, in which a line of intersection of the first surface and a horizontal plane extends, further in an upward direction than a center of gravity of a largest regular size sheet present in an initial position where the regular size sheet entering the tray is placed first. The supporting member supports a lower end portion of the largest regular size sheet and extends from inner sides to outer sides of end portions of the largest regular size sheet in the first direction.