Understanding the distribution and diversity of invasive alien species is increasingly crucial to meet legal requirements and guide effective management. Portugal currently hosts a large number of invasive alien plant species, with significant environmental and socio-economic impacts. However, the distribution of most of these species across the territory and the factors driving their spread and diversity remain largely unexplored. To address this, we present the first atlas of invasive alien plants in mainland Portugal. A total of 96 terrestrial and aquatic invasive alien plants are presented, encompassing all species listed under Portuguese national legislation and the European Union's list of invasive species of concern. Occurrence data were collected from a broad array of sources, including national and international biodiversity observation databases, citizen science data, literature, and data collections owned by managers and researchers—totaling approximately 85,000 records, with a mean of 879 records per species (range: 1–9,190). Each species was characterized based on multiple distributional parameters, and a k-means analysis was used to group species with similar distribution patterns. The richness of invasive plants was mapped at the municipality level. The drivers of their spatial variation were investigated using a comprehensive set of 30 variables representing multiple environmental and human factors. Results show that invasive plants are present in all mainland Portuguese municipalities but with high variability. Four main patterns of distribution were identified: species primarily located along the coast (e.g., Acacia saligna) and a gradient of species with narrow (e.g., Reynoutria japonica), moderate (e.g., Ipomoea indica), and wide distribution ranges (e.g., Cortaderia selloana). Invasive species richness was significantly higher in coastal and larger municipalities, particularly those closer to major urban centers and with a higher density of power lines. Our results provide the first comprehensive assessment of the distribution of invasive alien plants in mainland Portugal, establishing a much-needed baseline for future invasion prevention and management efforts.