A turning tool including a basic body having an insert seat, an indexable turning insert having a triangular basic shape, and a tightening element fixing the turning insert in the insert seat. The insert includes an upperside and an underside between which three side surfaces extend that are situated in a first imaginary, equilateral triangle and therefore converge in pairs at an angle of 60° toward corners in which cutting edges are included. The turning insert is engaged with the insert seat via an interface that includes elongate and straight, female-like and male-like engagement members, which are in engagement with each other in order to prevent turning of the turning insert. The insert seat includes a supporting surface having a corner section against which an active corner of the turning insert is applied. The insert seat includes a single active engagement member, which is spaced-apart from the supporting corner section, and is in engagement with one of three engagement members included in the turning insert and located in a second imaginary triangle having an equilateral shape, which is smaller than the first imaginary triangle and rotated in relation to the first imaginary triangle.