A hydrogen (H-2) gas sensor based on a silicon (Si) nanomesh structure decorated with palladium (Pd) nanoparticles is fabricated via polystyrene nanosphere lithography and top-down fabrication processes. The gas sensor shows dramatically improved H-2 gas sensitivity compared with an Si thin film sensor without nanopatterns. Furthermore, a buffered oxide etchant treatment of the Si nanomesh structure results in an additional performance improvement. The final sensor device shows fast H-2 response and high selectivity to H-2 gas among other gases. The sensing performance is stable and shows repeatable responses in both dry and high humidity ambient environments. The sensor also shows high stability without noticeable performance degradation after one month. This approach allows the facile fabrication of high performance H-2 sensors via a cost-effective, complementary metal-oxide-semiconductor (CMOS) compatible, and scalable nanopatterning method.