A sheet folding apparatus includes a conveyor, a first folding roller, a second folding roller, a third folding roller, a guide, a motor, and a driving force transmission mechanism. The motor causes the second folding roller to perform either forward rotation in which the second folding roller rotates to convey a sheet on a main conveying path in a conveying direction and to convey the sheet on a reflux conveying path from a joining position toward a branching position or backward rotation in a direction opposite to the forward rotation. The driving force transmission mechanism rotates the guide toward a first posture from a second posture in conjunction with the forward rotation of the second folding roller and rotates the guide from the first posture toward the second posture in conjunction with the backward rotation of the second folding roller.