A new species of Mammillaria is described: Mammillaria giselae, known from a single location in the Sierra de San Carlos, Tamaulipas, Mexico. A series of morphological characteristics helps to place the taxon in the Series Prolifera because of the form and size of the plant. It has no close relatives but shows a certain affinity to Mammillaria prolifera, but shape and form of the stems, lack of hairs or bristles in the axils, size and form and colour of the spines, colour and shape of the flowers as well as the structure clearly separate it from that species and its varieties. It is also compared to Mammillaria schiedeana var. dumetorum.