We report here on significant enhancement of the photochemical etching of p-type gallium phosphide (GaP) by Au plasmonic nanostructures. The photochemical etching rate of defect (dislocation) states of Au-coated p-GaP samples is ten times higher than blank samples when irradiated with 532 nm laser. It is confirmed that the enhancement of photochemical etching is wavelength selective. Only 532 nm laser can efficiently increase the photochemical etching rate, while lasers of other wavelengths (375, 405, 445, and 473 nm) show limited or negative effects. This observation can be attributed to defect (dislocation) enhanced photochemical etching through localized surface plasmon resonance of Au nanostructures. This method may open a new pathway for controlled fabrication of novel optoelectronic devices.