We report a flexible hydrogen sensing platform based on a single-strand yarn consisting of high-density electrospun nanofibers, on which nanograined Pd or Pd@Pt is coated via yarn spinning followed by sputter deposition. In general, Pd undergoes a phase transition to PdHx (alpha-PdHx at [H-2] < 1% and beta-PdHx at [H-2] > 2%), in which H atoms act as electron scattering centers, thus increasing the resistance. In our system, the sensors exhibit switchable H-2 sensing behaviors, that is, (i) Delta R/R-0 > 0 at [H-2] > 1% by the active electron scattering and (ii) Delta R/R-0 < 0 at [H-2] < 1% derived from nanograined Pd effects. Due to high mechanical stability stemming from nanogranular morphologies of Pd, which is essential for enduring a huge volume expansion upon exposure to high-concentration H-2, we could obtain a wide concentration range (4-0.0001%) H-2 detection resolution. Moreover, an ultrathin Pt overlayer coated on Pd offers an accelerated H-2 detection capability based on effective gas dissociation and activation properties. Furthermore, by virtue of the core (thread)-shell (nanofiber yarn) scaffold, long cycling reliability and flexibility were achieved. This facile and low-cost yarn fabrication method offers the development of single-strand thread-type wearable chemiresistors that possess a high surface area and open porosity, facilitating gas diffusion and reaction.