In the study presented here, we aimed to describe the epidemiological, clinical and pathological findings of 51 canine cases with histologically-verified diagnoses of primary cardiac hemangiosarcoma (HSA). The medical data for each dog, including signalment, presenting complaints, physical examination findings, results of various diagnostic testing performed and method of treatment, were checked. In addition, all 51 cases were re-examined pathologically. The tumor occurred most frequently in older Golden Retrievers, followed by Maltese dogs and Miniature Dachshunds. Mass lesions of HSA were found more commonly in the right auricle (RAu) (25/51) and right atrium (RA) (21/51), and the RA masses were significantly (P<0.001) larger than the RAu masses. The echocardiographic detection rate of masses in the RAu group (60%; 15/25) was significantly lower than that in the RA group (95%; 20/21). Survival time was significantly (P<0.05) longer for 5 dogs that received adjuvant chemotherapy after tumor resection than for 12 dogs that did not. In this series, the Maltese (9/51) and Miniature Dachshund (7/51), as well as the Golden Retriever, were represented more frequently than other breeds. The lower echocardiographic detection rate of RAu masses compared with RA masses may be related to tumor size and/or location. The significantly longer survival time for dogs receiving adjuvant chemotherapy indicates that postoperative chemotherapy could be useful for dogs with cardiac HSA.