Ovarian remnant sydrome is a well-known long-term complication of ovariectomy or ovariohysterectomy which is caused by the presence of an active ovarian tissue despite an elective surgery. As a result of leaving or dropping some ovarian tissue in the abdomen or because of an ectopic ovarian tissue, typical clinical signs of proestrous or oestrous, such as swollen vulva, proestral bleeding or behavioral changes develop. The diagnostic approaches are vaginal cytology, gonadal hormone measurements, ultrasonography and exploratory surgery. Surgical excision of the ovarian mass is the recommended treatment which is a difficult process. This paper describes the causes of the syndrome, the clinical presentation of the patient, the diagnosis and the treatment of ovarian remnant syndrome in dogs.