The garden dormouse is listed as “near threatened” on the IUCN Red List of threatened species. Research on this species concentrates primarily on their hibernating strategies and physiology. However, little is known about infectious entities in this species, even less about neoplastic diseases. This case series presents the first report of different neoplasms in a population of garden dormice (Eliomys quercinus) under human care. This report includes six animals (four males, two females, four to seven years old). Four animals were euthanized, and two were found dead in the enclosure. Tissues of the animals were stained with Hematoxylin and eosin. Special staining (methyl green pyronin, desmin, vimentin, and cytokeratin) was performed to confirm diagnoses. Tumors found in these animals consisted of a pancreatic adenocarcinoma, a plasma cell tumor, an osteosarcoma, and three sarcomas. Because of their declining population trend and efforts of reintroduction, knowledge about diseases is imperative, especially, if there might be a genetic influence, which could hinder the success of conservation efforts.