Functionalizing nickel nanoparticles with ligands such as phosphines can enhance their catalytic performance. However the ligand effects are poorly known at a molecular level. In this work, the adsorption properties of trimethylphosphine (PMe3) on Ni nanoparticles (NPs) and extended surfaces were investigated via spin-polarized dispersion-corrected Density Functional Theory calculations. The coverage effect of phosphine was explored by considering monoadsorption and monoshells on NPs, and various adlayers from low coverage to saturation on surfaces. Icosahedral, ino-decahedral, cubo-octahedral, truncated octahedral and Marks-decahedral Ni nanoclusters at size 146-147 atoms were compared to Ni(111), (100) and (110) surfaces. PMe3 strongly adsorbs on Ni surfaces and NPs, with a clear preference for icosahedron. The chemisorption on extended surfaces is globally stronger than the adsorption on NPs, demonstrating the necessity to describe NPs by explicit models. The topological analysis shows the formation of a Ni –P covalent dative bond with a net electronic donation from phosporous to nickel. The presence of strongly chemisorbed PMe3 weakens the total Ni magnetization by specifically quenching the spin polarization at Ni atoms bound to ligands. Our results invite the scientific community to explore the stability and reactivity of other phosphine ligands on metallic nanoparticles.