An experimental investigation has been carried out in a water channel on the transverse flow-induced vibration of a square cross-section cylinder at two different angles of incidence to the free-stream, α = 0° (square) and 45° (diamond). The oscillation amplitude and frequency response of the cylinder were measured over a range of reduced velocity, U*, and compared to the response of a circular cylinder. The phase of the lift force with respect to the cylinder's displacement was determined. Additionally, particle image velocimetry (PIV) was used to identify the transition of the wake structure from 2S to 2P mode for the diamond case. The results show that the square orientation follows a galloping response as expected, while the diamond behaves similarly to that of a circular cylinder undergoing vortex-induced vibration.