A single-sided, square-shaped indexable cutting insert has opposing square-shaped upper and lower surfaces and a peripheral side surface extending therebetween having four corner surfaces alternating with four relief surfaces. A corner cutting edge is formed at the intersection of the upper surface with each of the four corner surfaces. The lower surface includes a base surface from which a central boss and exactly four engagement ridges protrude. Each engagement ridge extends from the central boss to intersect with one of the four corner surfaces, and the base surface is divided into four base sub-surfaces. The insert may be clamped against a seating surface of a tool holder pocket solely by the engagement ridge located closest to the operative corner cutting edge making contact with a corresponding single engagement groove, and the two of the four base sub-surfaces making contact with two coplanar raised support surfaces.