To provide a sheet processing apparatus which performs fold reinforcing processing more properly than a conventional sheet processing apparatus having a fold reinforcing unit, and to provide a sheet folding method.

The sheet processing apparatus includes: a folding unit that folds a sheet bundle including a plurality of sheets to form a fold; a fold reinforcing roller that nips the fold of the sheet bundle having the fold formed thereon and conveyed to a fold reinforcing position, reciprocatingly moves along the direction of the fold, and reinforces the fold of the sheet bundle; and a heating member that heats the fold of the sheet bundle conveyed to the fold reinforcing position when the fold reinforcing roller reinforces the fold.