A 10-year-old female mixed-breed dog presented for acute dyspnoea developing nine months after multiple mammary tumours were identified. The dog was euthanased and postmortem examination revealed widespread embryonal rhabdomyosarcoma of the myotubular variant involving the largest mammary tumour, the spleen, kidney, liver, lung and heart. The diagnosis was confirmed by immunohistochemical detection of desmin and the electron microscopic demonstration of cytoplasmic myofilaments with early sarcomere differentiation and rudimentary z-lines within tumour cells. To the authors' knowledge, this is the first comprehensive case description of the myotubular variant of embryonal rhabdomyosarcoma in the dog. This case is also unusual due to the involvement of mammary gland, the old age of the dog at the time of diagnosis and the confirmation of metastasis in a dog diagnosed after two years of age.