Galvanic wet etching was adopted to fabricate Si nanowire arrays (NWAs) as a near-perfect subwavelength structure (SWS), which is an optically effective gradient-index antireflection (AR) surface and also exhibits super-hydrophobicity with an extremely high water contact angle (159°). Fresnel reflection and diffuse reflection over the broad spectrum can be eliminated by a Si NWA AR coating. Moreover, Si NWA SWSs show polarization-independent and omnidirectional AR properties. The wavelength-averaged specular and diffuse reflectance of Si NWA SWSs are as low as 0.06% and 2.51%, respectively. The effects of the surface profile of this biomimetic SWS on the AR and super-hydrophobic properties were investigated systematically.