A sheet feeder, which is configured to feed a sheet to a processing machine, includes a main body, a cutting unit, and an attaching portion. The cutting unit is attached to the main body and includes a cutter slidable in a first direction. The cutting unit is configured to cut, along in the first direction, the sheet to be fed to the processing machine with the cutter. The attaching portion is configured to attach the main body to the processing machine. The attaching portion is attachable to the shaft of the processing machine.