<p>The rapid expansion of global trade, tourism, and human mobility has increased the introduction of alien plants into new regions. Here, we assessed the role of plant characteristics and climatic suitability in the naturalization success of 1,407 cultivated alien plants in Southern Africa. We used mediation analysis with climate suitability as a mediator to quantify the direct and indirect effects of plant characteristics, including phylogenetic relatedness, seed mass, plant height, native origins, native range size, and growth forms on naturalization success. We found that naturalized species have higher climatic suitability compared to non-naturalized ones. Additionally, seed mass, plant height, short-lived herbaceous growth form, and native range size are positively associated with naturalization success. In contrast, phylogenetic relatedness and a native origin in Europe were negatively associated with naturalization success. These associations were indirectly mediated by climatic suitability, with indirect effects accounting for more than 30% of the total effect in all cases. Our study underscores the significance of considering the role of climatic suitability for a comprehensive understanding of how plant characteristics impact the naturalization success of alien plants.</p>