An indexable cutting insert (20, 120, 220), comprising: opposing upper and lower surfaces (22, 24) with a peripheral surface and an insert axis extending therebetween, the peripheral surface intersecting the upper surface to form a continuous upper peripheral edge, and including four insert side surfaces alternating with four insert corner surfaces, each insert side surface intersecting the upper surface to form a side cutting edge, and each insert corner surface intersecting the upper surface to form a curved corner cutting edge, wherein each side cutting edge includes a primary cutting edge adjoining one of the corner cutting edges at a first endpoint and a secondary cutting edge adjoining another one of the corner cutting edges at a second endpoint, wherein in a top view of the cutting insert (20, 120, 220) along the insert axis: the upper surface is square-shaped with each of the four primary cutting edges being straight, and the four primary cutting edges defining four sides of an imaginary first square, each primary cutting edge is tangential to its adjoining corner cutting edge, and each secondary cutting edge is continuously curved and entirely located in a corresponding one of four imaginary quadrants (Q1, Q2, Q3, Q4) defined by two mutually perpendicular imaginary lines bisecting the imaginary first square's four sides, and wherein: each insert side surface includes an insert side relief surface intersecting the upper surface, and an insert side abutment surface between the insert side relief surface and the lower surface, in a cross-section taken in a second plane perpendicular to the insert axis and intersecting the four insert side abutment surfaces, the four insert side abutment surfaces define four sides of an imaginary third square, characterised in that the imaginary first square is rotationally offset from the imaginary third square by a second offset angle (θ2). opposing upper and lower surfaces (22, 24) with a peripheral surface and an insert axis extending therebetween, the peripheral surface intersecting the upper surface to form a continuous upper peripheral edge, and including four insert side surfaces alternating with four insert corner surfaces, each insert side surface intersecting the upper surface to form a side cutting edge, and each insert corner surface intersecting the upper surface to form a curved corner cutting edge, wherein each side cutting edge includes a primary cutting edge adjoining one of the corner cutting edges at a first endpoint and a secondary cutting edge adjoining another one of the corner cutting edges at a second endpoint, wherein in a top view of the cutting insert (20, 120, 220) along the insert axis: the upper surface is square-shaped with each of the four primary cutting edges being straight, and the four primary cutting edges defining four sides of an imaginary first square, each primary cutting edge is tangential to its adjoining corner cutting edge, and each secondary cutting edge is continuously curved and entirely located in a corresponding one of four imaginary quadrants (Q1, Q2, Q3, Q4) defined by two mutually perpendicular imaginary lines bisecting the imaginary first square's four sides, the upper surface is square-shaped with each of the four primary cutting edges being straight, and the four primary cutting edges defining four sides of an imaginary first square, each primary cutting edge is tangential to its adjoining corner cutting edge, and each secondary cutting edge is continuously curved and entirely located in a corresponding one of four imaginary quadrants (Q1, Q2, Q3, Q4) defined by two mutually perpendicular imaginary lines bisecting the imaginary first square's four sides, and wherein: each insert side surface includes an insert side relief surface intersecting the upper surface, and an insert side abutment surface between the insert side relief surface and the lower surface, in a cross-section taken in a second plane perpendicular to the insert axis and intersecting the four insert side abutment surfaces, the four insert side abutment surfaces define four sides of an imaginary third square, characterised in that the imaginary first square is rotationally offset from the imaginary third square by a second offset angle (θ2). each insert side surface includes an insert side relief surface intersecting the upper surface, and an insert side abutment surface between the insert side relief surface and the lower surface, in a cross-section taken in a second plane perpendicular to the insert axis and intersecting the four insert side abutment surfaces, the four insert side abutment surfaces define four sides of an imaginary third square, characterised in that the imaginary first square is rotationally offset from the imaginary third square by a second offset angle (θ2).