The purpose of this study is to determine the presence/degree of sexual dimorphism of the 12th thoracic vertebra through a quantitative analysis and to further examine its potential and reliability in the sex estimation of human skeletal remains. This study also explores the age-related changes of human skeletal remains and how these affect morphological variation conducive to sex estimation. In order to assess this, the 12th thoracic vertebrae, femur and sacrum of 168 mature skeletal remains (94 males and 74 females) from the Raymond Dart Collection in Johannesburg, South Africa and 407 (205 males and 202 females) mature skeletal remains from the Hamann-Todd Collection in Cleveland, Ohio were analyzed. Only individuals whose group affiliation was designated as “South African Black” from the Raymond Dart Collection and “African American” from the Hamann-Todd Collection were measured. This was done to permit the examination of geographical variance within and between the two samples. The morphology of the 12th thoracic vertebra was examined by means of univariate and multivariate analyses to better assess each effect. These analyses resulted in relatively high correct classifications of males and females in all samples, with mean measurement values being larger in males in all measurements. While age-related changes have little effect on the high reliability of sex estimation in the African American sample, age-related changes decreases the reliability of sex estimation in the South African sample. Overall, this study reveals that the 12th thoracic vertebra has potential for use in sex estimation as a result of the skeletal morphological variation between males and females both documented in the Raymond A. Dart Collection of Human Skeletons and the Hamann-Todd Osteological Collection.