The phenomenon of interference between vortex-induced vibration (VIV) and galloping in the transverse degree of freedom was studied in the wind tunnel in the case of a spring-mounted slender rectangular cylinder with a side ratio of 1.5 having the short side perpendicular to the flow. The tests were carried out in a wide Scruton number range, starting from low values and increasing it in small steps by using eddy-current viscous dampers. This study helped understanding the dynamics of the interaction between the two excitation mechanisms and clearly highlighted the transition through four regimes of VIV-galloping interference. It was found that a high value of the mass-damping parameter is required to decouple the ranges of excitation of vortex-induced vibration and galloping completely, and for the quasi-steady theory to predict the galloping critical wind speed correctly. This conclusion is also relevant from the engineering point of view, as it means that structures and structural elements with ordinary mass-damping properties can exhibit sustained vibrations in flow speed ranges where no excitation is predicted by classical theories of vortex-induced vibration and galloping. Although most of the experimental tests were conducted in smooth flow at zero angle of attack, the paper also discusses the sensitivity of the results to a small variation of the mean flow incidence and to the presence of a low-intensity free-stream turbulence. •The interference of VIV and galloping was studied in the wind tunnel for a rectangular cylinder.•High Scruton number values are necessary for decoupling the two excitation mechanisms.•The transition with the Scruton number through various regimes of interference is described.•The applicability and the limits of the quasi-steady theory is discussed.•Sensitivity study to low free-stream turbulence and small angle of attack.•Low free-stream turbulence is found to boost the interference of VIV and galloping.