Provided is a sheet post-processing apparatus in which a control portion repetitively performs a width-direction alignment operation including a first alignment operation of moving width regulating members from standby positions to first regulating positions after a sheet that is carried in onto a processing tray has been aligned in a carry-in direction by abutting of an edge of the sheet against a reference plate, and a second alignment operation of moving the width regulating members from the first regulating positions to the standby positions after performing the first alignment operation, and then moving the width regulating members to second regulating positions. The sheet post-processing apparatus satisfies C1>C2>P1≥C3 where P1 is a size in a width direction of the sheet, and C1, C2, and C3 are intervals between the width regulating members at the standby positions, the first regulating positions, and the second regulating positions, respectively.