To provide a sheet folding device and an image forming system for stacking folded sheet and non-folded sheet which are not folded together without increasing a device size.

A post-processing machine includes a first folding part 4 that folds sheet orthogonally to a sheet conveying direction, a second folding part that folds orthogonally to the folded line made by the first folding part 4, a first discharge port 41 that discharges conveyed sheet without folding, a second discharge port 44 that discharges conveyed sheet after folding with the first folding part 4 only, a third discharge port that discharges conveyed sheet after folding with the first folding part 4 and the second folding part, and a sheet stack part 71 at an upper part of a folded sheet conveyance part 5 that conveys the sheet from the first folding part 4 to the second folding part, thus stacking the sheets discharged from the first discharge port 41 and the second discharge port 44 in the stack part 71.