Mammary gland tumors in dogs are very common in clinical practice. Betulinic acid is currently a compound considered to have anticancer properties in human mammary tumors via nanoemulsions. In this study, betulinic acid nanoemulsions with a particle size of less than 300 nm were prepared. Biopsies were obtained from five female dogs with mammary tumors for histopathological analysis, confirming that two were tubular mammary carcinomas (MMTs, malignant) and three were complex mammary adenomas (BMTs, benign). The five female dogs were administered with a daily oral dose of nanoemulsion containing 5 mg/kg of betulinic acid for 30 days. Tumor size was measured every 7 days, and the response to treatment was assessed according to RECIST (Response Evaluation Criteria In Solid Tumors) standards. In one of the females with MMTs treated with the nanoemulsion, the tumor size was reduced by approximately 38%, while in the BMT female dogs, the nanoemulsion reduced the tumor size by 25.3%. It was concluded that oral administration of betulinic acid nanoemulsions reduced the size of canine mammary tumors. Experimental studies are still needed to further evaluate this preparation.