A seat headrest includes: a pad assembly; a main cover surrounding a rear side of the pad assembly; an upper guide coupled to the main cover and provided with an upward/downward movement guide groove formed in the upper guide and elongated upward/downward; a post stay including an upper portion inserted in the upward/downward movement guide groove; and a post holding guide, in which the lower portion of the post stay is inserted and held and which is held to a seat back frame.