<p>The introduction of plant species beyond their native ranges has profound ecological and socio-economic consequences, particularly for grasses, which are frequently introduced and often highly successful invaders. This study analyses the diversity, distribution, and invasion dynamics of subspontaneous Poaceae across Western Europe. Using a comprehensive database compiled from specialised sources, 478 subspontaneous grass taxa were assessed, of which 46 were classified as invasive. Patterns in taxonomic richness, geographical origins, climatic affinities, residence time, and drivers of invasiveness were examined. Subspontaneous richness varied substantially among countries, with Belgium, the Netherlands, and France exhibiting the highest values, while invasive taxa generally represented a minor fraction (&lt; 10%) of the total Poaceae flora. Most taxa originated from Asia and the Mediterranean, with introductions occurring continuously from the sixteenth century, whereas invasive taxa displayed episodic historical peaks. Climatic analyses indicated that non-invasive taxa were introduced into regions closely matching their native climates, whereas invasive taxa exhibited broader climatic tolerance across multiple climate classes. Canonical correspondence analysis corroborated these findings, showing that invasive taxa occupy a wider climatic niche in introduced ranges. Generalised linear mixed models identified native climatic breadth, introduced range size, and residence time as significant predictors of invasiveness, with broader native climate tolerance and larger introduced ranges increasing invasion probability, and more recent introductions more likely to become invasive. Our findings highlight the interplay between climatic suitability and introduction dynamics in shaping the success of subspontaneous grasses, providing critical insights for risk assessment, monitoring and management of these taxa across Europe.</p>