Sexual size dimorphism (SSD) is prevalent among extant vertebrates and presumably was as common among fossil archosaurs. Here, we test the hypothesis that femur shape is a reliable indicator of sex among archosaurs using linear and geometric morphometric analyses on a large sample of sexed Alligator mississippiensis femora. Linear regression of femoral dimensions onto femur length in alligators shows that male femora are the largest and stoutest in midshaft and articular dimensions, whereas females show more variation in their midshaft dimensions. Using thin-plate splines analyses, regression of partial warps onto alligator femur length revealed that with increasing size the midshaft expands whereas shaft torsion diminishes, that femur shape differs according to sex, and that females show the maximum amount of shape variation at a size 72% that of the largest males. However, sexual dimorphism accounts for only 3% of the total femur shape variation in the sample. Differences in reproductive physiology (e.g, shelling eggs in females) may account for some of the fine morphological differences between males and females. However, size and individual variation appear to effect femur shape the most. These results have significant implications for inferring fossil archosaur sex from femoral dimensions and shape.