A 16-month old female Golden retriever dog was presented with a 2-week history of left-sided exophthalmos, third eyelid prominence, some scale of the trunk, mild anorexia, mild diarrhea and depression. In radiographic findings, a mass with increased opacity in the left retrobulbar area was observed. Cytologic and histopathologic examinations after enucleation and mass excision showed diffuse round cell accumulation and adjacent small area of elongated cell proliferation were shown. They also illustrated the neoplastic cells with large nucleoli, multiple nucleoli that differ in size and scant cytoplasm with several clear vacuoles. Extent of local invasiveness and central necrosis of the retrobulbar mass was identified by magnetic resonance imaging. Based on the physical and laboratory examination, radiographic, cytologic and histopathologic findings this case was diagnosed as extranodal malignant lymphoma of the retrobulbar area.