Hydrogen embrittlement (HE) is a ubiquitous and catastrophic mode of fracture in metals. Here, embrittlement is considered as an intrinsic ductile-brittle transition at the crack tip, where H at the crack tip can reduce the stress intensity K-Ic for cleavage below the value K-Ie required for ductile dislocation emission and blunting. Specifically, cleavage fracture along (111) planes in Ni occurs due to the formation of just 3 planar layers of H interstitial occupation at a sharp crack tip. During the cleavage process, the sub-surface H in the upper and lower layers can rapidly diffuse to the fracture surface, lowering the net fracture free energy to K-Ic < K-Ie and enabling brittle fracture. Details of the process are demonstrated using both first-principles density functional theory and a new interatomic potential for Ni-H. Thermodynamic and kinetic models show that the 3 layers of H can form at the crack tip in equilibrium at room temperature with bulk H concentrations and loading rates where H embrittlement in Ni is observed. The kinetic model also predicts the slow crack growth rate in agreement with experiments. The energetics of the mechanism is then shown to apply to cleavage along grain boundaries. All together, these results show that a version of "Hydrogen enhanced decohesion" is the operative embrittlement mechanism in Ni. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.