Ta3N5 is a very promising photocatalyst for solar water splitting because of its wide spectrum solar energy utilization up to 600 nm and suitable energy band position straddling the water splitting redox reactions. However, its development has long been impeded by poor compatibility with electrolytes. Herein, we demonstrate a simple sputtering‐nitridation process to fabricate high‐performance Ta3N5 film photoanodes owing to successful synthesis of the vital TaOδ precursors. An effective GaN coating strategy is developed to remarkably stabilize Ta3N5 by forming a crystalline nitride‐on‐nitride structure with an improved nitride/electrolyte interface. A stable, high photocurrent density of 8 mA cm−2 was obtained with a CoPi/GaN/Ta3N5 photoanode at 1.2 VRHE under simulated sunlight, with O2 and H2 generated at a Faraday efficiency of unity over 12 h. Our vapor‐phase deposition method can be used to fabricate high‐performance (oxy)nitrides for practical photoelectrochemical applications. Solar energy conversion: A simple and reliable vapor‐phase deposition technique is developed for fabricating GaN/Ta3N5 thin films that achieve state‐of‐the‐art solar water splitting performance and stability. The GaN overcoating strategy is readily applicable to stabilize various promising (oxy)nitrides for practical photo‐electrochemical applications.