A sheet folder includes a conveyance roller pair, first, second and third folding rollers, a guide, and circuitry. The conveyance roller pair conveys a sheet in a conveyance direction at a first speed. The first folding roller is downstream from the conveyance roller pair. The second and third folding rollers are in contact with the first folding roller to form first and second nips, respectively, with the first folding roller, and rotate along with the first folding roller. The guide is between the conveyance roller pair and the first nip in the conveyance direction, and is changeable between first and second postures. The circuitry is to control the guide to guide the sheet to the first nip and change the posture to the second posture to fold the sheet into the second nip, and control the first folding roller to rotate at a second speed and folds the sheet.

The sheet postprocessing device includes a sheet conveyance path, a sheet tray, a carry-in guide, a processing part, a discharge member, a blower device, and an intermediate conveyance part. The sheet tray includes an upstream-side stacking part. The intermediate conveyance part is provided at a position within a range from the upstream-side stacking part to the downstream-side stacking part in a conveyance direction, and allows a sheet to pass therethrough below the processing part. The intermediate conveyance part is made up by including an upstream part including a first clearance through which the sheet passes, and a downstream part placed on a downstream side of the upstream part and including a second clearance through which the sheet passes. A size of the second clearance in a planar direction perpendicular to a surface of the sheet is wider than a size of the first clearance in the planar direction.