Human activities and environmental changes have significantly increased the introduction of non-native species (NIS) into marine habitats, posing a challenge to biodiversity and ecosystem management. This study presents a comprehensive approach to assess the invasiveness of Phymactis papillosa, a recently introduced sea anemone on the Portuguese intertidal coast. An experiment was conducted to evaluate the salinity and temperature tolerances of this species and predict its potential distribution beyond its native range. A total of 270 individuals were used in a two-factor experimental design with groups of six individuals at five temperature treatments (15, 19, 23, 27, 31 °C) and three salinity conditions (32, 35, 38 psu) replicated three times. Mortality levels were measured after two weeks. Results showed that salinity had no impact on mortality, but temperature significantly affected survival. At 31 °C, all individuals died, at 27 °C mortality level was intermediate, and no mortality occurred at 23 °C or lower. These findings suggest that P. papillosa could potentially invade the Portuguese coast, the northern Iberian Peninsula, and the North Sea; however the high temperatures of the Mediterranean may be a limiting factor. Being confined to the rocky intertidal zone, P. papillosa could compete with native species, particularly with those from the same functional group. Aggressive interactions with six native sea anemone species were induced, with P. papillosa dominating three species from the genus Actinia and showing equivalent behaviour to A. sulcata. This behavioural dominance may negatively impact the spatial distribution of native Actinia species. The integration of these findings suggests that P. papillosa should be classified as an invasive species with the potential to significantly impact benthic communities along the Portuguese and northern Atlantic intertidal coasts.