Electrolysis of seawater can be a promising technology, but chloride ions in seawater can lead to adverse side reactions and the corrosion of electrodes. A new transition metal boride‐based self‐supported electrocatalyst was prepared for efficient seawater electrolysis by directly soaking nickel foam (NF) in a mixture of phenolic resin (PR) and boron carbide (B4C), followed by an 800 °C annealing. During PR carbonization process, the reaction of B4C and NF generated nickel boride (NixB) with high catalytic activity, while PR‐derived carbon coating was doped with boron atoms from B4C (B−CPR). The B−CPR coating fixed NixB/B4C particles in the frames and holes to improve the space utilization of NF. Meanwhile, the B−CPR coating effectively protected the catalyst from the corrosion by seawater and facilitates the transport of electrons. The optimal NixB/B4C/B−CPR/NF required 1.50 and 1.58 V to deliver 100 and 500 mA cm−2, respectively, in alkaline natural seawater for the oxygen evolution reaction.