SummaryA case of giant cell tumour of bone (GCTb) in the lung and in a subcutaneous mass located in the right flank, with a probable primary origin in the mid‐diaphysis of the right tibia, was described in a 8‐year‐old female cat. Numerous multinucleated giant cells were homogeneously distributed among a population of ovoid or spindle‐shaped mononuclear cells. All of them were positive for vimentin suggesting a mesenchymal origin. Spindle‐shaped tumour cells resemble fibroblastic cells, showing collagen fibres in their vicinity. Ovoid mononuclear cells are similar to macrophages, with a cytoplasm rich in electron‐dense lysosomes. Multinucleated giant cells appear morphologically similar to osteoclasts. These findings are supported for the positive reaction to tartrate‐resistant acid phosphatase (TRAP) and lysozyme, encountered only in ovoid and multinucleated giant cells. No immunoreactivity against human oestrogen receptors was observed in the nuclei of any neoplastic cells.