Exotic animal invasions constitute a major threat to biodiversity worldwide. Once the Italian wall lizard (Podarcis siculus) was accidentally introduced in Lisbon (Portugal) two decades ago, the present assessment determined the core range occupied by this alien lizard in the urban area of Parque das Nações. Despite the apparent current scenario of non-expansion, the alien species interferes with the local distribution pattern of the native P. virescens, with both lizard species shown to use the available microhabitats differently. The native P. virescens population shows an overall heterogeneous distribution in the urban matrix. Otherwise, the occupancy of the invasive species clusters within the first area of introduction (garden in front of the Lisbon Oceanarium), as well as in adjacent gardens. There, P. siculus competitively excludes the native P. virescens. Apparently, there is no known reason there that prevents the alien lizard from colonizing more available geographic area and expanding.