Biodiversity is severely declining worldwide and one of its main threats is invasion by exotic species. Successful management of these species requires citizens to be aware and engaged to prevent new introductions and support control interventions. We used questionnaires to investigate the knowledge and perceptions of Portuguese citizens aware of environmental issues, regarding invasive alien plants (IAPs). The influence of formal training in environmental areas vs. informal education about IAPs on the knowledge and perceptions of respondents was analysed. The results (735 responses) showed that environmentally conscious citizens have a great knowledge and awareness of IAPs (and biodiversity). While most respondents realize that IAPs negatively alter the landscape and cause negative impacts, some recognize that they can also have positive impacts and benefit the landscape. Formal training in environmental areas influences perception and increases knowledge about IAPs. In general, the opinion of whether a species should be removed or not was greatly influenced by the recognition of its invasive status, and less so by whether respondents liked or considered the species beautiful, suggesting that if citizens are informed about the invasive status of a species they are more willing to support its removal. More surprisingly, our results suggest that knowing the “Invasive Plants in Portugal” (Invasoras.pt) project, used as a proxy for higher awareness/informal education, increases knowledge and alters perceptions. Besides increasing understanding of IAPs' knowledge and perceptions among citizens, our work show that investing in public awareness projects pays off and can be focused on improving the planning of IAPs management strategies.