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The trend towards an increased incidence of marine heatwaves (MHWs) due to climate change poses a serious threat to the health of coastal ecosystems. Here, we characterized MHWs based on daily in situ water temperature measurements collected from 1998 to 2019 in two intertidal coves, Oleiros and La Franca, located in the central Cantabrian Sea coast, N Spain. The two study locations cover the transition zone between cold- and warm-temperate benthic communities that characterizes the biogeography of intertidal assemblages along the SW Bay of Biscay. We examined the incidence, duration and magnitude of MHWs, and assessed the emergence of trends towards more frequent, prolonged and intense MHWs during the study period. We further assessed mechanistic links with modes of climate variability by examining the association between MHW occurrence and the East Atlantic pattern (EA). We detected 78 MHW events between Jan. 1998 and Mar. 2019 (40 in Oleiros, 38 in La Franca) and more than half were synchronous among study locations. The incidence, duration, and intensity of MHWs were higher during the positive phase of the EA, which is associated with air pressures above normal and southwesterly winds during summer. The recorded MHWs also coincided with documented shifts in the abundance and distribution of dominant macroalgae species that triggered abrupt changes in the structure of intertidal communities of the region. Our findings and the present scenario of climate change emphasize the need to enhance research around the trends in the occurrence of MHWs in the coast of northern Spain, and the NE Atlantic Ocean coast in general. [Display omitted] • We analyzed marine heatwaves at two coastal locations in the southern Bay of Biscay. • In situ measurements reveal a steady increase in sea surface temperature. • The frequency and length of marine heatwaves did not significantly change over time. • The positive phase of the East Atlantic pattern favors marine heatwaves. • Marine heatwaves match reported population shifts in coastal macroalgae. [ABSTRACT FROM AUTHOR]