Abstract: The present disclosure relates to a transition metal-doped nickel phosphide nanostructure, a method for preparing the same, and a catalyst for electrochemical water decomposition including the transition metal-doped nickel phosphide nanostructure. More specifically, a transition metal-doped nickel phosphide nanostructure can be prepared by converting a zinc oxide nanostructure grown on a substrate vertically by hydrothermal synthesis to a transition metal-doped nickel oxide nanostructure by cation exchange and then phosphorizing the nickel oxide. The transition metal-doped nickel phosphide nanostructure of the present disclosure is advantageous in that it has superior catalytic activity and conductivity due to large surface area. In addition, when used as a catalyst for water decomposition under an alkaline condition, it has a low overvoltage and can have excellent catalytic activity for hydrogen evolution reaction or oxygen evolution reaction.