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; a driving circuit that drives the fold reinforcing roller along the direction of the fold; and a controller that controls the number of times of the reciprocating movement of the fold reinforcing roller, on the basis of a value of an electric current of the driving circuit flowing when the fold reinforcing roller that starts driving from a standby position rides over an end of the sheet bundle.