The Aegean archipelago has long been the main research area of numerous biogeographers, plant ecologists and taxonomists due to its intricate palaeogeography and high environmental and topographical heterogeneity. Nevertheless, some parts of this archipelago are essentially unexplored and the processes driving spatial variation in species composition remain unaddressed. Aiming to fill these gaps, we investigated the flora and plant diversity patterns of the Northern Sporades island group, as well as its biogeographical relationships. The study area lies in the biogeographical region of the West Aegean islands and comprises 23 islands and islets. The total flora of the study area consists of 1202 infrageneric taxa, belonging to 517 genera and 120 families, reflecting its geographical and bioclimatic characteristics. The endemic element consists of 41 taxa (3.4% of the flora), eight of which are restricted to the West Aegean islands and two are single island endemics. Area emerged as the most important variable in shaping plant species richness, while niche-based processes played a lesser role in driving these patterns. Regarding the taxonomic and phylogenetic beta-diversity patterns, environmental filtering and not dispersal limitation seems to shape the plant assemblages of the Northern Sporades islets. Biogeographically, the Northern Sporades island group seems to be closer connected to the Kiklades rather than to Evvia or the adjacent mainland, due to their longer isolation and separate palaeogeographical history during the Quaternary.