The article is devoted to the scientific and organisational activities of Hermann Sommer (1899–1962), the founder and head of the Office for the Care and Preservation of the Cultural Heritage in the Fischhausen district of the German province of East Prussia, during the difficult period of Germany’s history from 1929 to 1945. It describes the circumstances surrounding the creation, as well as the later rescue and finally rediscovery by the archaeological community, of Sommer’s historical and archaeological legacy. One of the most important components of the archaeological part of the heritage is the Fischhausen Archive, a card-index archive of archaeological monuments that were known in the district in question by 1945. By this time, the first experience of using the data from the archive had already demonstrated the enormous potential of these documents for the reconstruction of the prewar state of research, as well as for the modern study of the archaeological sites on the Kaliningrad Peninsula. The search for the rest of his legacy has already resulted in a number of unexpected discoveries of further archaeological material. Preliminary results also indicate that similar archives of archaeological monuments could also have been created for other districts of the former German province of East Prussia.