Surface plasmon (SP) excitations in metals facilitate confinement of light into deep-subwavelength volumes and can induce strong light-matter interaction(1,2). Generally, the SP resonances supported by noble metal nanostructures are explained well by classical models, at least until the nanostructure size is decreased to a few nanometres, approaching the Fermi wave-length lambda(F) of the electrons(3-6). Although there is a long history of reports on quantum size effects in the plasmonic response of nanometre-sized metal particles(7-12), systematic experimental studies have been hindered by inhomogeneous broadening in ensemble measurements(3), as well as imperfect control over size, shape, faceting, surface reconstructions(13), contamination(14), charging effects(15) and surface roughness(16) in single-particle measurements. In particular, observation of the quantum size effect in metallic films and its tuning with thickness has been challenging as they only confine carriers in one direction. Here, we show active tuning of quantum size effects in SP resonances supported by a 20-nm-thick metallic film of indium tin oxide (ITO), a plasmonic material serving as a low-carrier-density Drude metal(17-21). An ionic liquid (IL)(22,23) is used to electrically gate and partially deplete the ITO layer. The experiment shows a controllable and reversible blue-shift in the SP resonance above a critical voltage. A quantum-mechanical model including the quantum size effect reproduces the experimental results, whereas a classical model only predicts a red shift.