An eight-year-old entire female boxer was presented with a two-week history of anorexia and lethargy and two-day history of unilateral left epistaxis. Clinical findings and laboratory test results suggested disseminated intravascular coagulation. On blood smear evaluation, occasional large epithelioid-like unclassified cells were detected. Occasionally these cells were organised in small clusters. Bone marrow examination revealed a marked infiltration by a malignant population of the same epithelioid-like cells. The dog was euthanased because of the guarded prognosis. Following histology and immunohistochemistry, a widespread undifferentiated carcinoma of unknown primary origin was diagnosed. To the authors’ knowledge, this is the first case of carcinoma cell leukaemia reported in a dog. Carcinoma cell leukaemia is a rare oncological condition previously described in humans, characterised by non-haematopoietic neoplastic cells in peripheral blood.