To provide a sheet processing apparatus that reinforces folding more properly than a conventional sheet processing apparatus having a fold reinforcing unit, and to provide an image forming apparatus and a sheet folding method.

The sheet processing apparatus includes: a folding unit that forms a fold line by folding a sheet bundle; a fold reinforcing unit that nips the fold line formed by the folding unit, reciprocates in a fold direction and has a fold reinforcing roller for reinforcing the fold line of the sheet bundle; and a controller that times a fold reinforcing time required for the fold reinforcing roller to once reciprocate and when the fold reinforcing time exceeds a predetermined threshold, increases the number of reciprocation of the fold reinforcing roller.