Knowledge of sex ratios of species with temperature-dependent sex determination (TSD) is key to provide baseline information which can be used to inform management strategies and predictions of how climate change can affect populations. In marine turtles, increased incubation temperatures can lead to extreme biases in sex ratios and reduced hatching success. Here we present a long-term analysis (34 years) of incubation durations of leatherback turtles (Dermochelys coriacea) in Brazil, the only population of this species breeding in the Southwest Atlantic, and estimate offspring sex ratios. We deployed data loggers recording nest temperatures in 2015, 2016, 2017 and 2019 (n = 28 clutches), to predict offspring sex ratios based on incubation temperatures during the thermosensitive period when sex is determined. The overall mean incubation duration for leatherback turtle clutches in Brazil (1988-2021) was 66.3 days (range 52-91, SD = 6.4, n = 867), decreasing by 4.4 days between the first and last 10 years of monitoring and varying latitudinally across the nesting range of the population. When modelled to the overall nesting season and accounting for nesting seasonality, we estimated the current (2012-2021) mean season-wide primary sex ratio to be 66.5% female (range 55.4% to 90.2%). Hindcasting for the first ten years of monitoring (1988-1997) showed the average predicted offspring sex ratios would have been 58.4% female (range 41.3-79.9). This population has not shown a phenological shift in the timing of nesting over the period 1988-2021. These findings suggest that, although the primary sex ratio of this population has likely become more female-biased in recent decades, the spatial and temporal variation observed demonstrates resilience to the effects of increasing temperatures under climate change.