A water cluster ion beam was irradiated on a poly(methyl methacrylate) (PMMA) surface to examine the possibility of applying the water cluster ion beam technique to the surface processing and modification of polymers. The sputtering yields of PMMA substrates irradiated with water cluster ion beams increased with acceleration voltage and dose of the water cluster ion beam. The threshold acceleration voltage of sputtering was approximately 3 kV. The X‐ray photoelectron spectroscopy (XPS) analysis of the PMMA surface irradiated with the water cluster ion beam suggested the degradation of the PMMA side chains. The XPS spectrum of the surface of the sputtered particle catcher at 45° backward direction showed approximately the same shape as the XPS spectrum of the PMMA surface irradiated with the water cluster ion beam.