Cantabrian mixed forests, located in areas of Spain, Portugal, and France, serve as an essential biogeographic transition region, noted for its extraordinary plant diversity and ecological intricacy. To aid conservation and research initiatives, a uniform checklist of vascular plants was created, incorporating information from citizen science platforms, scientific databases, herbarium records, and local floras. The outcome is a carefully selected collection of more than 8000 taxa, with over 76% recognized as native, highlighting the area’s importance as a reservoir of biodiversity and a climate refuge. Taxonomic discrepancies were resolved via expert verification and adherence to international naming standards, establishing a dependable basis for ecological research. The checklist demonstrates notable variations in organisms, ecological approaches, and evolutionary lineages, influenced by geographical diversity, climate variations, and past land use patterns. Importantly, the study emphasizes the drawbacks of unchecked biodiversity data and shows the benefits of expert-driven synthesis for addressing gaps and biases in species documentation. The floristic information presented here can act as a basis for transboundary conservation planning, ongoing biodiversity tracking, and the development of adaptive management approaches in response to climate change and ecological decline. This initiative represents an important move towards safeguarding the distinct natural heritage of this distinctive biogeographic region.