This study focuses on the distribution and impact of the invasive species tree-of-heaven (Ailanthus altissima (Mill) Swingle) in the easternmost county of the Republic of Croatia. The research was conducted from May to September 2022 and 2023, utilizing the systematic random sampling method. The presence of tree-of-heaven was recorded in both rural and urban environments, where only the number of trees having a medium tree trunk diameter at breast height (DBH) of 10-20 cm and a small DBH of 5-10 cm was significantly higher in rural compared to urban sites. The negative effects of the tree-of-heaven population are due to its vigorous clonal growth (biological characteristics; p < 0.001) and dense monoculture stands, resulting in decreased plant species richness and native species diversity (ecological impact; p < 0.001. The harmful effects of invasion depend on the site where plants grow. Still, serious damage, based on significant factor loadings (eigenvalue >1), was recorded not only on environmental but also on functional, safety, and aesthetic aspects, causing economic loss and mechanical damage.