This dataset contains quantitative records of marine mollusc epifaunal assemblages associated with the brown macroalga Halopteris scoparia collected from shallow subtidal habitats around São Miguel Island, Azores, Portugal, during July 2020. A total of 40 macroalgal samples were collected by SCUBA diving across four coastal locations. Associated molluscan fauna were sorted, identified to species level, and quantified. The dataset includes abundance records for 16 mollusc species (14 gastropods and 2 bivalves), together with measurements of algal dry biomass. The dataset supports analyses of species richness, abundance patterns, and community composition of macroalgal-associated mollusc assemblages in shallow coastal habitats. It provides a valuable ecological baseline for studies addressing biodiversity patterns, species responses to environmental change, and the structure of Atlantic island coastal ecosystems. The data were collected within the framework of the 3B-vent project (Biodiversity, Biological Interactions and Biotechnological Products of Coastal Hydrothermal Vents in the Azores), which aimed to advance knowledge of marine biodiversity, species interactions, and ecosystem functioning in Azorean coastal environments.