A treatment instrument and a treatment system capable of both a chipping treatment and a blood stanching treatment in arthroscopic surgery is provided. A treatment instrument includes a probe comprising a treatment portion which treats a treatment target part by ultrasonic vibration, and a back portion provided on a side opposite to the treatment portion； a hollow sheath which surrounds the probe, the sheath having a cutout made to expose the treatment portion, and a protrusion which is provided in the sheath to cover the back portion on a side opposite to the cutout and which has dimensions smaller than the dimensions of the probe in a direction that intersects with a central axis of the probe.
