An optimal sex ratio is arguably one of the most important demographic traits of species. Rising global temperatures threaten temperature-dependent sex determination (TSD) species with extreme sex ratio bias and ultimately extinction. Because sex steroid hormones can impact sex determination in TSD reptiles, variation in their maternal transfer within the egg yolk may form a buffer mechanism against raising temperatures. We tested this hypothesis by quantifying the effect of maternal oestradiol (E2) and testosterone (T) on offspring sex in a threatened TSD population of loggerhead sea turtles (Caretta caretta). Circulating levels of E2 and T in nesting females, in egg yolks at oviposition and in neonates were measured. Immediately after oviposition, nests were relocated into an in situ experimental hatchery where temperatures were controlled by standardising the incubation depth. We used affinity propagation clustering on hormone profiles guided by incubation duration, to sex individuals from these nests in a non-lethal manner, offering a novel conservation tool for this endangered species. Despite standardised temperatures, we found a large level of variation in the sex ratio of clutches, which correlated in a non-linear manner with maternal investment of the E2:T ratio in egg yolks. Males were produced at equal levels of E2 and T and females were produced on either side of this optimum. Modelling sex ratios for the coming century, we show that maternally-derived hormones form a trans-generational mechanism of TSD plasticity that adjusts offspring sex ratios in endangered sea turtles.