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Dioecy is a rare sexual system among angiosperms whose occurrence and frequency depend on the kind and geographical location of the ecosystem. We evaluate the frequency of dioecy in the Caatinga, a special semi-arid vegetation type in Brazil. We compare the floral traits and other ecological characteristics usually related to dioecy between the Caatinga and other tropical ecosystems. Dioecious species of the Caatinga were compiled based on 24 published floristic lists including most of the Caatinga domain. These lists were compared with publications comprising data on sexual systems in different kinds of vegetation in the world. Sexual system confirmation and the database of the reproductive traits of the dioecious species were made by consulting herbaria, literature, and field observations. We registered 22 dioecious species occurring in the Caatinga. The overall dioecy frequency was 2.36 %, considering all Caatinga species, and 6.91 % within woody species. Most dioecious species exhibited traits related to this sexual system (e.g., woody habit, greenish inconspicuous flowers, generalistic pollination syndrome, and zoochory). The distribution of dioecy across the Caatinga plant community exhibited a very low level of phylogenetic signal ( D = −0.145), indicating a phylogenetic structure under Brownian evolution ( p = 0.671). The frequency of dioecy in the Caatinga (2.36 %) proved lower than reported for angiosperms in general (ca. 6 %) and for other tropical ecosystems. This low frequency can be related to particular ecological characteristics of the Caatinga, such as low percentage of trees >3-4 m tall (13-18.7 %), low rainfall (50 % of the Caatinga vegetation with annual rainfall lower than 750 mm), and strong seasonality (6-9 months of dry season). Dioecious species of the Caatinga exhibited typical reproductive traits for this sexual system, similar to what has been reported for dioecious species of other tropical ecosystems in the world. [ABSTRACT FROM AUTHOR]