Several factors are suggested to be involved in the development of nontraumatic inguinal hernias (NTIHs) in dogs, but case series studies focusing on the etiology and treatment outcomes are limited. The aim of this study was to retrospectively evaluate the clinical characteristics and treatment outcomes of NTIHs in dogs. Medical records of 42 dogs with surgically treated NTIHs were reviewed. Forty-one dogs were included in the study, all dogs were small breeds weighing <10 kg, and middle to older age (>5 yr old; 33 cases), female sex (34 cases), and miniature dachshunds (26 cases) predominated. Left-sided occurrence was common (30 left, 9 right, 2 bilateral), and organ protrusion was seen in 22 cases (15 uteri, 9 small intestines, 1 colon). Fourteen of 15 uterine herniations (93%) were located left side. Ovariohysterectomy was performed with herniorrhaphy in 27/30 intact bitches, two of whom also underwent resection and anastomosis of a devitalized portion of the small intestine. Recurrence was seen in only one male dog. These results suggest that NTIHs are more likely to occur in small-breed female dogs, and that age may increase the risk of left-sided uterine protrusion; however, the long-term results after herniorrhaphy with ovariohysterectomy are excellent.