In the present study, ZnO thin film was grown on nano-porous silicon by atomic layer deposition (ALD) whereas porous silicon was prepared by a stain etching method for three different durations, 4 min (PS1), 8 min (PS2) and 12 min (PS3). SEM analysis shows that ZnO nanoparticles with a size of 20–50 nm were uniformly distributed on nano-porous silicon. AFM analysis shows that the surface roughness of the nanoporous silicon increases continuously with the increase of porous silicon etching time. In contradiction, the surface roughness is almost equal for ZnO/PS1, ZnO/PS2 and ZnO/PS3. XRD analysis shows that the ZnO nanoparticles exhibited a hexagonal wurtzite structure. XPS characterization was used to analyze the chemical composition and states present in the ZnO coated porous silicon. The DRS UV-Visible absorbance spectrum reveals that ZnO/PS3 very strongly absorbs visible light around 526 nm. ZnO coated porous silicon, especially ZnO/PS3, exhibited higher photocatalytic activity compared to ZnO coated glass towards methylene blue dye degradation. Likewise, the negative biased ZnO/PS3 exhibited superior photocatalytic activity compared to unbiased and positive biased ZnO/PS3. The enhanced solar photocatalytic and photoelectrocatalytic activity is attributed to the visible light absorption of ZnO/PS and the effective injection of photogenerated electrons from porous silicon to ZnO even if porous silicon is not directly involved in any redox reactions.