The feeding ecology and distribution of the Common Kingfisher Alcedo atthis has been extensively studied during the breeding season in European freshwater habitats, but there is much less known about the ecology of this species in marine and brackish habitats. This study aims to (1) document the presence of Common Kingfishers in marine habitats of Galicia (NW Spain), based on a database of bird sightings collected in mainland Galicia (2004–2020) and on the Cíes Islands (Coast of Galicia; 2008–2019), and (2) study their diet in marine habitats, analyzing 17 pellets collected during autumn and winter in a coastal salt lagoon on the Cíes Islands. During autumn and winter, the number of observations of the species in mainland Galicia were higher in marine habitats than in freshwater habitats (χ²₁ = 10.88, P < 0.001). Moreover, Common Kingfishers visit the Cíes Islands mostly during autumn and winter (98.6% of observations). Both datasets show that marine habitats are very important for wintering birds. In the coastal lagoon on the Cíes Islands, the Common Prawn Palaemon serratus was the most frequent prey (41.7%), accounting for the highest percentage of the total biomass (68%); much more than the second most frequent prey, gobies (Gobiidae; 35.5%). This finding is exceptional in Europe, since only a few studies report shrimp as a prey item for Common Kingfishers, probably due to the lack of data from marine habitats during winter.