A cost-effective and scalable approach was developed to produce monodisperse NiCoP nanocrystals (NCs) with composition tuned over the entire range (0 ≤ x ≤ 2). NiCoP NCs were synthesized using low-cost, stable and low-toxicity triphenyl phosphite (TPP) as a phosphorus source, metal chlorides as metal precursors and hexadecylamine (HDA) as a ligand. The synthesis involved the nucleation of amorphous Ni-P and its posterior crystallization and simultaneous incorporation of Co. The composition, size and morphology of the NiCoP NCs could be controlled simply by varying the ratio of Ni and Co precursors and the amounts of TPP and HDA. Ternary NiCoP-based electrocatalysts exhibited enhanced electrocatalytic activity toward the hydrogen evolution reaction (HER) compared to binary phosphides. In particular, NiCoP electrocatalysts displayed the lowest overpotential of 97 mV at J = 10 mA cm and an excellent long-term stability. DFT calculations of the Gibbs free energy for hydrogen adsorption at the surface of NiCoP NCs showed NiCoP to have the most appropriate composition to optimize this parameter within the whole NiCoP series. However, the hydrogen adsorption energy was demonstrated not to be the only parameter controlling the HER activity in NiCoP.