The parametric decay instability of two high‐frequency surface plasma waves, which arises when an intense electromagnetic plane wave is incident on a semi‐infinite, unmagnetized plasma from a vacuum, is investigated on the basis of the hydrodynamic equations for warm electrons. It is shown that the instability occurs only if the incident wave is of the transverse magnetic type. The growth rate for the instability and the threshold amplitude of the incident wave are determined. In contrast to the usual ’’two‐plasmons decay instability’’ in an infinite, unmagnetized plasma, the present ’’surface‐decay instability’’ is possible even if the incident pump field is spatially homogeneous.