Transition metal-doped nickel phosphide nanoparticles with metallic properties are prepared by a simple and facile wet-chemical method. It is shown for the first time that these transition metals: iron, cobalt, manganese, and molybdenum, can atomically substitute nickel in the parent hcp phosphide lattice as a single phase without significant change in its metallic structure and morphology. They are employed as electro- and photocatalysts for hydrogen evolution reaction, which show highly tunable activities dependent on electron filling of their metallic bands and H coverage according to our experimental and theoretical rationalizations. Molybdenum-doped nickel phosphide nanoparticle with lower H coverage exhibits the best hydrogen evolution performance in electrocatalytic hydrogen evolution reaction, which also shows excellent photocatalytic hydrogen production with organic photosensitizer. In addition, cobalt-doped nickel phosphide nanoparticle with higher H coverage with aqueous photosensitizer gives more superior hydrogen evolution rate.