This dataset is a catalogue of seal images that were collected as part of the former Countryside Council for Wales (CCW) and Natural Resources Wales' (NRW) seal monitoring and evidence work. Images were also provided by volunteers, partners, organisations and members of the public through collaboration with CCW/NRW. The images are of heads, necks and flanks of grey seals (mostly female) photographed at seal haul out sites around the Celtic and Irish Seas from 1992 to 2016 and show the pelage markings of seals that can be individually recognised/identified from subsequent photographic encounters – photo-identification (Capture, Mark, Recapture). The database contains over 17,000 photographic images of more than 9,000 grey seals. Photo-ID allows individual seals to be captu red through time and space and analyses of the data provide insights into site fidelity and associations, movements and connectivity, and longevity/survival.