Solar‐to‐hydrogen conversion based on photocatalytic and photoelectrocatalytic water splitting is considered as a promising technology for sustainable hydrogen production. Developing earth‐abundant H2‐production materials with robust activity and stability has become the mainstream in this field. Due to the unique properties and characteristics, transition metal phosphides (TMPs) have been proven to be high performance co‐catalysts to replace some of the classic precious metal materials in photocatalytic water splitting. In this Minireview, we summarize the recent significant progress of TMPs as cocatalysts for water splitting reaction with high activity and stability. Firstly, the characteristic of TMPs is briefly introduced. Then, we mainly discuss the recent research efforts toward their application as photocatalytic co‐catalysts in photocatalytic H2‐production, O2‐evolution and photoelectrochemical water splitting. Finally, the catalytic mechanism, current existing challenges and future working directions for improving the performance of TMPs are proposed. What's all the phos about? This Minireview concerns recent progress in metal phosphide‐based heterogeneous photocatalysts for water splitting to produce high‐purity hydrogen. Various systems with optimized or enhanced photocatalytic efficiency are introduced through specific examples. The possible catalytic mechanism, recent challenges, and future research directions for optimizing the performance of metal‐phosphide‐based co‐catalysts are discussed.