Ni and its alloys are susceptible to hydrogen embrittlement. In this work, we perform a systematic density functional theory (DFT)-based investigation of hydrogen trapping at different types of defects in Ni. With focus on the grain boundary (GB) segregation and clustering of H atoms, we investigate segregation energy profiles of H at special coincidence site lattice coherent GBs in Ni, initial stages of H-cluster formation and their dependence on volume expansion/contraction. Further, the embrittlement of Ni by hydrogen segregation to GBs and its effect on the reduction of the cohesive strength of GBs as well as of the bulk are investigated based on the hydrogen-enhanced decohesion (HEDE) mechanism. The influence of a number of alloying elements on H segregation and HEDE in Ni is calculated and analyzed by means of a physical model. Finally, the results are used to draw preliminary conclusions on the effect of H on HEDE in Ni based alloys.