We present a new type of nanoporous antireflection (AR) coating based on grass-like alumina with a graded refractive index profile. The grass-like alumina AR coating is fabricated using atomic layer deposition (ALD) of alumina and immersion in heated deionized water. Optical transmittance of 99.5% at 500 nm was achieved with average transmittance of 99.0% in the range of 350–800 nm at normal incidence for double-sided coated glass. Angular spectral transmittance (0–80°) of the double-sided AR coated glass was also measured in the range of 350–800 nm and found to have mean spectral transmittance of 94.0% at 60°, 85.0% at 70°, and 53.1% at 80° angles of incidence, respectively. The grass-like alumina AR coating is suitable for mass production with the presented technique: even hundreds of optical components can be coated in parallel. Furthermore, as an ALD-based technique, the coating can be deposited conformally on surfaces with extreme topography, unlike many spin-coating, physical vapor deposition or glancing angle deposition-based coatings used today.