The international wildlife trade is a lucrative business. Although a huge variety of animal groups are trafficked, the Psittaciformes (parrots) are amongst the most traded avian groups. Deliberate or accidental releases of imported parrots have led to the establishment of feral populations in many countries. Far from their native habitats, parrots may cause economic and ecological damage, and may even favour the transmission of zoonotic diseases. Despite this, the links between numbers of imported individuals and the establishment of non-native populations is not well known. In this study, we analysed data on imports of two well-known invasive parrots, the Monk Parakeet Myiopsitamonachus and the Ring-necked Parakeet Psittacula krameri, in Spain. We contrasted this information with the growth of known naturalised populations of these species from 1975 to 2015 and compared the success of these two species with other Psittaciformes imported in similar numbers into the country. We show that more than 190,000 Monk Parakeets were imported from Uruguay and Argentina, and almost 63,000 Ring-necked Parakeets were legally brought into the country from Pakistan and Senegal. For both species, wild populations grew exponentially following peak importation periods in 2015 (18,980–21,455 Monk Parakeets and 3,005–3,115 Ring-necked Parakeets). Even though imports of the two species were banned in Spain in 2005, wild populations are now self-sustaining. We argue that these parrot populations started from accidental and deliberate bird escapes, especially from birds originally captured in the wild. Although lack of more precise data makes it difficult for us to propose clear statistical associations between imports and established bird populations, we nonetheless suggest that the international trade is with some certainty the main cause for the origin of naturalised populations of invasive species in Spain. Our conclusions are useful to help manage similar animal groups that are numerous in the wildlife trade, especially for wild-caught social species.—Lucrecia Souviron-Priego, Antonio Román Muñoz, Jesús Olivero, J. Mario Vargas & John E. Fa. (2018). The legal international wildlife trade favours invasive species establishment: the Monk and Ring-neked Parakeets in Spain. Ardeola, 65: 233–246.