Sexual dimorphism, the condition whereby males and females differ from one another physically, is one of the most fundamental aspects of the biology of any animal. However, sexually dimorphic characters can be subtle and are mainly related to soft tissue anatomy. They are, therefore, difficult to identify reliably in the fossil record particularly when dealing with small sample sizes and osteology alone. The first geometric morphometric analysis of dimorphism in a thyreophoran (armored) dinosaur shows that the femora of the stegosaurian dinosaur Kentrosaurus aethiopicus (Upper Jurassic, Tanzania) bear a statistically significant shape difference of the proximal end, which is independent of size and is therefore proposed to be a sexual difference. Although the disarticulated nature of the material means that intraspecific variation in other skeletal elements, such as the enigmatic dermal armor, cannot be identified as sexual dimorphism at this time, this study provides a methodology for further work on articulated stegosaurian specimens and has the potential to reveal additional information regarding the paleobiology and population dynamics of this poorly understood clade.