Coastal grassland vegetation records from São Miguel Island (Azores) and the south-western coast of mainland Portugal.

Background: The present work provides an inventory of the plant species recorded in two distinct coastal grassland vegetations: the Azores Archipelago (São Miguel Island) and the south-western coast of mainland Portugal (Sesimbra and Sines Regions - Setúbal District). Sites were selected in both regions to have a similar general substrate (rocky), latitude and elevation. Thirty-one sites were selected in the coastal grasslands: thirteen were located on São Miguel Island and eighteen on the mainland, distributed across Sesimbra (12) and Sines (6). All sites were visited once during the spring of 2022. In each site, 30 photos were taken at 5-metre intervals, for a total of 930 photos.The Sesimbra and Sines Regions were chosen because they represent well-preserved examples of coastal grassland vegetation on the south-western coast of mainland Portugal, sharing similar environmental characteristics with the coastal grasslands of São Miguel Island, such as rocky substrate, Atlantic exposure and comparable latitude. This design allows a meaningful comparison between insular and continental vegetation under similar abiotic conditions, providing a standardised framework for documenting coastal plant diversity across contrasting geographic contexts. New information: Most records consolidate previously documented occurrences from national repositories, but here they are re-validated through standardised, plot-based, georeferenced surveys that harmonise taxonomy and metadata. Crucially, this release constitutes the first coordinated, standardised inventory of coastal grassland vascular plants spanning the Azores and the south-western mainland of Portugal, enabling direct cross-regional comparisons. In addition to several within-plot distributional updates, we document multiple introduced and invasive taxa, most notably