A 29-yr-old female western lowland gorilla (Gorilla gorilla gorilla) was evaluated for low fertility and a midterm abortion. Laboratory testing included karyotyping, which revealed an unusual mosaicism for Turner syndrome with Triple X (47,X/49,XXX). This appears to be the first report of Turner syndrome in a great ape. In humans, Turner syndrome occurs in approximately 1 in 3,000 females, with half of those monosomic for the X chromosome. A small proportion is mosaic for a triple X cell line (3–4%). In humans, Turner syndrome is associated with characteristic phenotype including short stature, obesity, a broad chest with widely spaced nipples, webbing of the neck, and anteverted ears. This individual gorilla is significantly shorter in stature than conspecifics and is obese despite normal caloric intake. Individuals with Turner syndrome should also be screened for common health issues, including congenital heart defects, obesity, kidney abnormalities, hypertension, hypothyroidism, and diabetes mellitus. Animals with decreased fertility, multiple miscarriages, fetal losses, unusual phenotypes, or a combination of these symptoms should be evaluated for genetic abnormalities.