A sheet folding device includes: a sheet receiving unit with a pair of guide plates to restrict opposite sides of one or a plurality of stacked sheets and a striking portion to be struck by leading ends of the introduced sheets; a roller group to introduce the sheets into the sheet receiving unit, strike the introduced sheets on the striking portion to form a bent portion on the sheets, and nip and fold the formed bent portion and discharge the sheets from the sheet receiving unit; and a curve forming unit to form a curved portion on the sheets, at a position closer to the leading ends of the sheets than a position where the bent portion to be folded by the roller group is formed, the curved portion being curved in a width direction perpendicular to a direction of transfer of the sheets.