Understanding hydrogen-metal interactions is critical for developing refractory complex concentrated alloys (CCAs), applicable to the hydrogen economy. In this study, we revealed a hydrogen-assisted spinodal decomposition phenomenon at the nanoscale in an equiatomic TiNbZrHfTa CCA upon its exposure to H₂ at 500 °C. Such a decomposition pathway was characterized by a periodic compositional modulation with an up-hill diffusion behavior of the principal metallic elements, particularly Zr, over an extended treatment period (from 0.5 h to 2 h) in an H₂ atmosphere, probed by three-dimensional atom probe tomography. Consequently, the decomposed alloy consisted of a needle-shaped phase enriched in Zr and Ti and a phase enriched in Nb and Ta. Crystallographically, the spinodal features aligned preferentially along 〈001〉 directions of the matrix phase to minimize elastic strain energy. To better understand the role of hydrogen in spinodal decomposition, a statistical thermodynamic model was further developed by incorporating hydrogen to predict the phase stability of the TiNbZrHfTa-H system. This analysis suggested that hydrogen destabilizes the single solid-solution phase by expanding the spinodal region. Such nanoscale spinodal decomposition enhanced the hardness and anti-abrasive properties of the investigated alloy. Thus, this study not only provides fundamental insights into the effect of hydrogen on phase stability, but also demonstrates a novel alloy design strategy by introducing hydrogen as an interstitial alloying element to tailor the microstructure.