An alternative method to retrofit reinforced concrete (RC) columns with insufficient shear reinforcement is investigated. The retrofit involves external prestressing of the columns in the transverse direction to increase both shear strength and drift capacity. External post-tensioned clamps, consisting of high-strength steel rods connecting a set of steel angles, were applied around the columns at different spacings and initial post-tensioning stresses. The tension induced in the steel rods exerts lateral confining pressure on the column by bearing of the angles against the corners of the column. Ten RC columns furnished with external post-tensioned clamps were tested under cyclic loads and approximately constant axial loads. In addition, six RC beams with clamps were tested under monotonically increasing loads. Both the column and beam specimens were fabricated with no transverse reinforcement in the form of conventional steel ties. Therefore, the external clamps were the only source of reinforcement resisting shear. The lateral prestress provided by the clamps was observed to increase the shear stress at the formation of the first inclined crack and at failure. As a result, the mode of failure of columns vulnerable in shear changed from shear failure to a more ductile failure dominated by flexure. The observed increase in shear strength is dependent on the lateral prestress and the tensile strength of the concrete. A simple equation, based on the mechanics of materials, is presented to calculate the shear strength of RC columns with external prestressing.