A 14-year-old neutered female Jack Russell terrier was presented with a history over the past several years of afreely movable subcutaneous mass on the right lateral thigh, that was considered to be a lipoma. Shortly beforepresentation, a mastectomy had been performed. On histopathological examination of the inguinal lymph node excisedduring the procedure, numerous mast cells were identified. Fine-needle aspirates of the mass on the thigh were collected,and cytology confirmed that it was not a lipoma but a mastocytoma. After staging the tumor, it was treated withneoadjuvant prednisolon, followed by the surgical removal of the mass with radical margins. The defect was closed witha transposition flap. Histopathology and immunohistochemistry indicated a grade-II mastocytoma without negativeprognostic factors. The dog recovered well after surgery and 347 days after treatment, there were no signs of recurrence.