The developmental success of marine ectotherms depends heavily on temperature, an important environmental factor, particularly during early life stages that are highly sensitive to thermal stress. In this study, we investigated how Tripneustes gratilla embryos developed at different temperatures (20°C, 25°C, 28°C, 30°C, and 35°C) throughout their early developmental stages, which include fertilization, cleavage, morulation, blastulation, gastrulation, and larval formation. Cumulative proportion of success over time (CPS, proportion-hour) estimates were fitted against temperature in a non-linear thermal reaction norm curve using the Lactin-2 model variant to determine key thermal traits, including the maximum CPS (CPS_max), thermal optimum (T_opt), critical thermal limits (CT_min and CT_max), thermal tolerance range (TTR), and thermal safety margin (TSM). Results revealed optimal development between 25°C and 28°C, with reduced success and developmental arrest at thermal extremes. Importantly, thermal tolerance narrowed and T_opt increased with developmental progression, indicating increased sensitivity in later stages. The research demonstrates that thermal windows exist at specific developmental stages, and the larval stage shows the narrowest tolerance range and safety margin. Our results highlight the vulnerability of T. gratilla's early development to ocean warming and underscore the need to identify thermal thresholds to inform conservation strategies for marine invertebrates under climate change.