This paper presents an experimental investigation of the onset of cavitation over a square-section cylinder. A numerical simulation of noncavitating steady incompressible turbulent flow around the cylinder was also performed. Three square-section cylinders of different sizes have been tested in a water tunnel. Cavitation inception has been detected visually. Observations of cavitation inception showed that the first visible traces of cavitation appear in the form of small bubbles in the free shear layer at the leading edge corners of the cylinders suggesting that the small-scale vortices in the shear layer over the side faces and associated pressure fluctuations are responsible for the onset of cavitation. The cavitation inception indices have been found to decrease with increasing the freestream velocity. In addition, the results showed that the variation of the cylinder size has no effect on the cavitation inception indices.