A number of large-growing species of Agave L. (Agavaceae) have become firmly established as part of the introduced flora of parts of the Algarve, southern Portugal, particularly close to human settlements. Two species, which include at least four taxa, Agave americana L. ssp americana, A. americana var marginata Trel., A. salmiana Otto ex Salm-Dyck var salmiana and A. salmiana var ferox (K. Koch) Gentry, recorded in southeastern Portugal are briefly discussed here, their residence status is determined, and a key to aid in their identification and that of other cultivated forms of Agave americana is provided. In addition, a putative hybrid between A. americana ssp americana and A. salmiana var salmiana is recorded here for the first time. The nomenclature of the naturalized species of Agave is updated to reflect current usage. The four naturalized Agave taxa are compared to A. franzosinii Baker, which is widely cultivated along the Mediterranean coast of Europe.