A new thermodynamic assessment of the Ni-H system has been carried out, providing a complete description valid up to 6 × 109 Pa of this system at the center of hydrogen storage issues. The study includes the hydride formation reaction and the presence of a miscibility gap in the fcc interstitial solid solution of hydrogen in nickel. In addition to a complete literature review, first-principles calculations were carried out and combined with other modeling approaches. The cluster expansion method allowed us to describe the energetic interactions between atoms leading to the miscibility gap in the fcc solid solution. Besides, the compressibility at high pressure was characterized for each phase including the condensed phases. For this purpose, a specific high-pressure model was assessed with the contribution of quasi-harmonic phonon calculations. The obtained consistent model allows us to characterize entirely the thermochemical behavior of the Ni-H system.