Provided are: a magnesium hydroxide particle that emits little alpha-rays and shows excellent flowability and dispersibility when added to a resin; a resin composition containing the magnesium hydroxide particle; and a method for manufacturing the same. A magnesium hydroxide particle that is a hexagonal column-shaped particle, the external shape of a crystal of which consists of two (i.e., top and bottom) hexagonal base planes, said base planes being parallel to each other, and six outer peripheral column planes formed between these basal planes, wherein the hexagonal column-shaped particle has a length in the c-axial direction of 0.5-6.0 microns, the length in the c-axial direction corresponds to at least 50% of the average particle diameter of the hexagonal column-shaped particle, the alpha-ray emitting quantity is 0.020 c/cm2/h or less, and the purity is 98.0% or more, and a method for manufacturing the same.