Mammillaria hahniana (old lady cactus) is a species of flowering plant in the family Cactaceae, native to central Mexico. It grows to 25 cm (10 in) tall by 50 cm (20 in) broad. The solitary spherical stems, 12 cm in diameter, are covered in white down and white spines. Reddish purple flowers are borne in spring and summer, sometimes forming a complete ring around the apex of the plant.