Closed-loop control of vortex-induced non-resonant vibration of a flexible square cylinder isexperimentally investigated in this paper. Piezo-ceramic actuators were embedded insidethe cylinder to cause an oscillation of the cylinder surface, which subsequently altered thefluid–structure interaction. Experiments were conducted in a wind tunnel at Reynolds numbers,Re, of 2800 and 8000. Two typical control schemes,i.e. Y_control and u+Y_control, were deployed using feedback signals from structural vibrationY and combinedY and fluctuatingflow velocity u, respectively. The control effects on the structural vibration and flow wereassessed using a laser vibrometer, an optical fiber Bragg grating sensor,hot wires and particle image velocimetry. Experimental results show thatboth vortex shedding from the cylinder and the vortex-induced non-resonantvibration were effectively suppressed. The best control effects were observed withu+Y_control in use; the root meansquare values of Y, thestructural strain rate εy along the lift direction and u, i.e. Yrms, εy,rms andurms, and thecirculation Γ dropped at Re = 8000 by 58%, 52%, 53% and 88%, respectively, compared with the unperturbed case. It wasfound that the control effectively modified the nature of the fluid–structure interaction bychanging the in-phase fluid–structure synchronization at all dominant frequencies intoanti-phase interaction, accounting for the suppression in both vortex shedding andstructural vibration.