Quasi‐1D cadmium chalcogenide quantum rods (QRs) are benchmark semiconductor materials that are combined with noble metals to constitute QR heterostructures for efficient photocatalysis. However, the high toxicity of cadmium and cost of noble metals are the main obstacles to their widespread use. Herein, a facile colloidal synthetic approach is reported that leads to the spontaneous formation of cadmium‐free alloyed ZnSxSe1−x QRs from polydisperse ZnSe nanowires by alkylthiol etching. The obtained non‐noble‐metal ZnSxSe1−x QRs can not only be directly adopted as efficient photocatalysts for water oxidation, showing a striking oxygen evolution capability of 3000 µmol g−1 h−1, but also be utilized to prepare QR‐sensitized TiO2 photoanodes which present enhanced photo‐electrochemical (PEC) activity. Density functional theory (DFT) simulations reveal that alloyed ZnSxSe1−x QRs have highly active Zn sites on the (100) surface and reduced energy barrier for oxygen evolution, which in turn, are beneficial to their outstanding photocatalytic and PEC activities. Nearly monodisperse alloyed semiconductor quantum rods (QRs) are formed from polydisperse nanowires by means of etching of alkylthiol. The obtained noble‐ and heavy‐metal‐free alloyed QRs demonstrate striking oxygen evolution capability and enhanced photo‐electrochemical activity due to the highly active Zn sites on their (100) surface and reduced energy barrier for water oxidation, as revealed by the density functional theory simulations.