All but one of the more than 1900 species in the cactus family (Cactaceae) are endemic to the Americas. Several species were introduced to Europe on the return trip from the first voyage to the New World in the 15 th century. Since then, cactus species have fascinated botanists, plant enthusiasts and collectors. Today cacti can be found all over the world, and several species of Cactaceae, introduced either deliberately or accidentally outside their native range, have become invasive. Horticulture has increased greatly in importance as a pathway of introduction for new cactus species worldwide, and is now virtually the only pathway for the global movement of cacti. Hundreds of new ornamental cacti are being introduced by seeds outside their native range every year. Reliable ways of identifying invasive taxa are urgently needed. We studied the seed morphology of 293 cactus species to determine its significance as an invasive character. Seeds were obtained from nurseries in the Netherlands, Spain and Germany. The seed collection represents about 57 cactus genera and provides a good representation of cactus species in the international ornamental trade. A significant variation was observed in seed morphology (form, colour, brilliance, surface, size, and weight). The seeds of cactus species known to be invasive worldwide fit into a basic type characterized by a light brown colour, large size, rounded shape, rough surface and large mass. Some exceptions do exist. Overall, seed morphology seems to be a good character to use in screening cacti for invasiveness.