Invasive alien plants are generally known to possess strong competitive abilities, which contribute to the biodiversity loss in the recipient communities. This study aimed to evaluate the competitive ability of a focal invader Lactuca serriola and its impact on neighboring plants within its introduced range, South Korea. Through a combination of field research and greenhouse experiments, we examined species diversity, individual performance, and leaf functional traits of L. serriola-invaded and uninvaded plots across nine habitats between June 2022 and August 2022. The findings revealed that species diversity and the performance of most co-occurring species exhibited no significant differences between invaded and uninvaded plots. Also, invaded plots displayed significantly lower soil total nitrogen and available phosphorus compared to uninvaded plots. To establish a clear cause-and-effect relationship based on the results of field research and to confirm the interactions between the focal invader and neighboring species, a pairwise competition experiment was conducted. This experiment involved manipulating competition and nutrient levels as treatments and comparing performance of individuals in intraspecific and interspecific competition within each species. The results demonstrated that L. serriola was not a relatively strong competitor compared to its neighboring species. In high-nutrient environments, the performance of neighboring species was significantly higher in interspecific competition with L. serriola, while L. serriola exhibited higher growth in intraspecific competition than interspecific competition. The high specific leaf area (SLA) and low leaf dry matter content (LDMC) of L. serriola indicated that the invader would get an advantage in invasion through rapid growth rather than suppressing co-occurring species. However, in low-nutrient condition, both intraspecific and interspecific competitive outcomes of L. serriola were comparable. Additionally, L. serriola showed the highest root-shoot ratio among species in low-nutrient condition, specifically being higher in interspecific competition than in intraspecific competition. These results suggest that although L. serriola did not excel as a competitive suppressor, its successful invasion can be attributed to its relatively higher stress-tolerance to nutrient-poor environments compared to co-occurring species.