Like many other terns, European Sandwich Terns Thalasseus sandvicensis exhibit a period of post-breeding dispersal before migrating to their wintering grounds in Africa. To understand the significance of Danish waters for these movements, we analysed changes in the abundance and distribution of Sandwich Terns throughout Denmark from late June to October using our own count data as well as data from the online bird portal DOFbasen. To understand the geomorphology of, and threats to important roosting sites, we assessed their habitat characteristics and the level of (human) disturbances using data recorded in the field. We used 2422 resightings of individually marked juvenile Sandwich Terns to assess the use of Danish waters by terns of different geographical origin. The results showed that the temporal use of roosting sites differed between the western and eastern part of Denmark, with maximum numbers of Sandwich Terns occurring earlier in the west. Important roosting sites were characterised by sandy shallow coastlines, which reflect suitable habitats for their key food species. It seems most likely that phenological patterns in Sandwich Terns reflect the phenology and relative abundance of their food items. Results from the resighting data suggested that the proximity of breeding colonies to the different post-breeding areas in Denmark influenced the choice of post-breeding area as well as the phenological patterns. Large roosting numbers in combination with resightings of Sandwich Terns dispersing from various European countries highlight the international importance of Danish waters during the period of post-breeding dispersal. Observed average disturbance rates exceeding one disturbance per hour underlines the need for protected roosting locations on beaches with high human activity.