This letter reports the experimental evidence of a 2.3 eV blue shift of the plasmon loss lines observed in the x-ray photoelectron spectra of an ultrathin Si layer equivalent to 1.5×1015  at./cm2 deposited onto a randomly oriented Al2O3 single crystal. The plasmon line shifts, which is roughly proportional to the deposited Si(mass)−2/3, are attributed to quantum confinement effects in agreement with electron energy loss measurements performed on nanosized Si particles.