A sheet folding device includes a transport unit that transports a sheet; an abutting unit that abuts an end of the sheet being transported to form a bent portion at the sheet; a folding unit that nips the bent portion of the sheet by a pair of folding rollers to form a fold line in the sheet; a guide member that partially covers the folding roller and guides the sheet; and a position adjustment unit that adjusts a position of the guide member. An exposure amount of the folding roller with respect to the sheet can be varied by adjusting the position of the guide member through the position adjustment unit.