The search for solar‐driven photocatalysts for overall water splitting has been actively pursued. Although metal oxynitrides with metal d⁰/d¹⁰‐closed shell configuration are very promising candidates in terms of their visible light absorption, they usually suffer from serious photo‐generated charge recombination and thus, little photoactivity. Here, by forming their solid solutions of LaTaON₂ and CaTaO₂N, which are traditionally considered to be inorganic yellow‐red pigments but have poor photocatalytic activity, a class of promising solar‐driven photocatalysts La_1‐xCa_xTaO_1+yN_2‐y (0 ≤ x, y ≤ 1) are explored. In particular, the optimal photocatalyst with x = 0.9 has the ability of realizing overall water splitting with stoichiometric H₂/O₂ ratio under the illumination of both AM1.5 simulated solar light and visible light. The modulated key parameters including band structure, Ta bonding environment, defects concentration, and band edge alignments revealed in La_0.1Ca_0.9TaO_1+yN_2‐y have substantially promoted the separation of photogenerated charge carriers with sufficient energetics for water oxidation and reduction reactions. The results obtained in this study provide an important candidate for designing efficient solar‐driven photocatalysts for overall water splitting.