A sheet transport device includes a transport roller, a drive source, and a gear train. The transport roller transports a sheet. The gear train transmits rotating force from the drive source to the transport roller. The gear train includes a movable gear and a leaf spring. The movable gear is configured to move between a normal position where transmission of the rotating force to the transport roller is enabled, and a disengaged position where the transmission of the rotating force to the transport roller is interrupted, and also to move in an axial direction, and includes a helical gear that generates force to axially move in one direction, when the movable gear moves from the normal position to the disengaged position. The leaf spring buffers the movement of the movable gear in the one direction.