Mammillaria gigantea is a species of cactus in the subfamily Cactoideae native to Mexico. It is named for its large size.
Mammillaria gigantea is a cactus that grows low and almost cake-shaped, often slightly sunken at the top, and covered in white wool felt. Its blue-green body is high and in diameter, with densely packed warts that produce milky juice. The axillae are covered with white wool. It has up to 12 small, fine-needle, straight white radial spines, each up to long, and 4 to 6 strong, usually curved central spines, with the lowest one reaching up to . New central spines are yellow-brown with dark tips, later becoming yellowish, white to horn-colored, and reddish at the base.
As with all Mammillaria, the flowers appear in a wreath. They are green-yellow and about in diameter. The fruits are pink to greenish, and the seeds are brown.
Mammillaria gigantea is found in the mountainous regions of the Mexican states of Guanajuato, Durango, San Luis PotosÃÂ, and Querétaro, at altitudes between .
It was first described in 1898 by Karl Moritz Schumann. The specific epithet "gigantea" comes from Latin and means "enormously large," referring to the plant's size.