Lead‐free halide perovskites have triggered interest in the field of optoelectronics and photocatalysis because of their low toxicity, and tunable optical and charge‐carrier properties. From an application point of view, it is desirable to develop stable multifunctional lead‐free halide perovskites. We have developed a series of Cs2PtxSn1−xCl6 perovskites (0≤x≤1) with high stability, which show switchable photoluminescence and photocatalytic functions by varying the amount of Pt4+ substitution. A Cs2PtxSn1−xCl6 solid solution with a dominant proportion of Pt4+ shows broadband photoluminescence with a lifetime on the microsecond timescale. A Cs2PtxSn1−xCl6 solid solution with a small amount of Pt4+ substitution exhibits photocatalytic hydrogen evolution activity. An optical spectroscopy study reveals that the switch between photoluminescence and photocatalysis functions is controlled by sub‐band gap states. Our finding provides a new way to develop lead‐free multifunctional halide perovskites with high stability. A series of Cs2PtxSn1−xCl6 perovskites with high water stability demonstrate switchable photoluminescence and photocatalytic functions on variation of the amount of platinum. With low platinum contents, the photocatalytic function is activated and photocatalytic hydrogen evolution is achieved in water, whereas with high platinum contents the photoluminescence function is achieved.