Single‐walled carbon nanotubes decorated with Pd nanoparticles on thin poly(ethylene terephthalate) sheets serve as building blocks for the fabrication of mechanically flexible hydrogen sensors (left frame of the figure) with excellent sensing performance. The sensitivities (i.e., the change of resistance) of the as‐fabricated sensors are comparable to (or even higher than) the sensors built on rigid substrates and the responses are reversible (right frame of the figure).