Mammillaria hahniana, the old lady cactus, is a species of flowering plant in the family Cactaceae, native to central Mexico.

Reddish purple flowers are borne in spring and summer, sometimes forming a complete ring around the apex of the plant.

Like all cacti it must be kept dry in winter, and fed and watered during the active growth cycle in summer.

Increasing quite readily and flowering at a relatively early age, it is a suitable subject for domestic cultivation.

[3] It has gained the Royal Horticultural Society's Award of Garden Merit.