Provided are a flame retardant which is good in dispersibility in an organic polymeric material, and does not lower, even after mixed with an organic polymeric material, material properties thereof; a flame retardant composition; and a shaped body. The flame retardant of the present invention includes magnesium hydroxide particles subjected to a surface treatment, the particles satisfying the following (A) to (D): (A) an average particle size is 2 μm or less according to a laser diffraction method; (B) a BET specific surface area is 3 to 15 m2/g; (C) a degree of mono-dispersion is 50% or more, the degree of mono-dispersion being represented by the following equation: Degree of mono-dispersion (%)=(average primary particle size [μm] of particles according to SEM observation/average particle size [μm] of particles according to laser diffraction method)×100; and, (D) just after 1 mL of a 0.1 M solution of nitric acid is dropwise added at a dropping rate of 0.1 mL/minute to a suspension obtained by adding 1.0 g of the flame retardant to 100 mL of an aqueous mixed solution containing 0.1% by weight of each of a wetting agent and an electrolyte, pH of the suspension is 9.0 or less according to a potentiometric titration.