To provide a sheet processing apparatus which accurately adjusts a position of a bundle of sheets according to various processing, and an image forming system.

The sheet processing apparatus includes a pair of support plates 61A and 61B, a moving mechanism 62, holding units 53 and 54, and a controller. The pair of support plates 61A and 61B come in contact with the inside of a fold of a folded bundle of sheets S2. The moving mechanism 62 moves the pair of support plates 61A and 61B along to the direction to which the fold between the pair of support plates 61A and 61B, and the bundle of sheets S2 faces. The holding units 53 and 54 hold the bundle of sheets S2 from the thickness direction of the bundle of sheets S2. The controller controls the moving mechanism 62 to move the pair of support plates 61A and 61B to a position corresponding to the thickness of the bundle of sheets S2 before the holding units 53 and 54 hold the bundle of sheets S2.