This dataset contains the values of natural abundance of stable nitrogen isotopes in bulk tissues of intertidal macroalgae (Phaeophyceae) collected between 1989 and 2023 along the NW coast of Spain. Macroalgal species included Fucus vesiculosus, F. spiralis, F. serratus, and Bifurcaria bifurcaria, depending on their availabilty at the sampling sites. Stable isotopes were determined in the upper 3 cm of algal thallus. Stable isotope determinations were made by isotope-ratio mass spectrometry using an elemental analyzer (bulk samples; Viana et al. 2011). This dataset has been assembled by combining samples stored in the environmental specimen data banks of the Universidad de Santiago de Compostela (BEAG) and in the specimen collection of the Centro Oceanográfico de A Coruña (COAC), Spain.