Invasive alien species pose significant threats to global biodiversity and ecosystems. Given the high importance of context-dependency in invasion dynamics, testing fundamental hypotheses of invasion ecology in different environmental contexts is crucial to understand the underlying processes of the establishment and spread of alien species. This study focuses on the invasive alien Senecio inaequidens DC. (South African ragwort), investigating the interplay between abiotic and biotic factors influencing its invasion success. Through field surveys across three semi-natural habitats in Northern Italy, we assessed the relative importance of abiotic (soil characteristics) and biotic (resident vegetation diversity and similarity to the target alien) variables on S. inaequidens performance. Our results showed a pivotal role of biotic interactions over abiotic conditions in determining the success of the alien species. We found evidence of biotic resistance by resident plant communities, and it was mainly explained by resident species’ diversity and cover. However, when analysing the similarity patterns between S. inaequidens and the invaded communities, we surprisingly found a negative relationship between S. inaequidens success and similarity to resident communities, indicating a better performance of the alien species when growing with more similar species. Abiotic factors had only marginal effects on influencing the strength and direction of biotic interactions and partially showed a stronger biotic resistance (in terms of resident species richness) in more benign abiotic conditions (soil with higher nutrient concentrations and in habitats less disturbed and stressed). Therefore, S. inaequidens appears to be more susceptible to competition rather than adverse abiotic conditions, making it a good colonizing species rather than a strong competitor, which might imply that in ruderal or stressed environments being functionally similar to the recipient communities could confer this species an advantage to invade. Our findings underscore the complexity of invasion dynamics, highlighting the need to consider diverse mechanisms, including taxonomic, phylogenetic, and functional measures, as well as the abiotic context, to inform effective management strategies for invaded ecosystems.