Abstract 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 Cs 2 Pt x Sn 1− x Cl 6 perovskites (0≤ x ≤1) with high stability, which show switchable photoluminescence and photocatalytic functions by varying the amount of Pt 4+ substitution. A Cs 2 Pt x Sn 1− x Cl 6 solid solution with a dominant proportion of Pt 4+ shows broadband photoluminescence with a lifetime on the microsecond timescale. A Cs 2 Pt x Sn 1− x Cl 6 solid solution with a small amount of Pt 4+ 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.