The present invention provides a transition metal phosphate and a production process thereof, a positive electrode, and a sodium secondary battery. The transition metal phosphate contains sodium (Na), phosphorus (P) and a transition metal element and having a BET specific surface area of 1 m2/g to 50 m2/g. The process for producing a transition metal phosphate comprises steps (1) and (2): (1) a step of bringing a phosphorus (P) source, a sodium (Na) source, an M source (M is one or more transition metal elements) and water into contact with each other, and obtaining a liquid material thereby, and (2) a step of separating water from the liquid material and obtaining a transition metal phosphate thereby.