© 2018 The Zoological Society of London Vertebrates display a wide variety of sex-determining mechanisms (SDMs). Identifying SDMs for different taxa is important for understanding the origins and phylogenetic history of sex-determining mechanisms. Temperature-dependent sex determination (TSD) is found in several vertebrate and invertebrate species, and has been studied most extensively in reptiles. The initial discovery of TSD was in a lizard, the African redhead agama (Agama agama), over 50 years ago. The original study of A. agama used only two incubation temperature treatments and had a small sample size, thus it could not fully characterize the pattern of TSD in this species. The goal of this study is to revisit this research to better characterize the pattern of TSD. We also assessed the effect of fluctuating incubation temperatures on offspring sex ratio to provide insight into how variation in offspring sex ratio is produced under natural conditions. After incubating eggs across six constant temperature treatments, we demonstrated that females are produced at relatively cool temperatures and males at warmer temperatures (30°C); fluctuating thermal regimes showed this same pattern. However, some females are again produced at even warmer temperatures, resulting in relatively balanced sex ratios toward the end of the sex-determining reaction norm. Though the shape of the sex-determining reaction norm shown here is unusual, it shows some similarities to that of other agamid lizards. Future studies that explore the diversity of TSD reaction norms and underlying physiological mechanisms will provide new insights into the evolution of TSD.