Anthropogenic climate change is a significant threat to marine ecosystems, and hexacorals—including reef-building corals and sea anemones—are particularly vulnerable to rising ocean temperatures. However, a promising mechanism of acclimatization to ocean warming is hormesis, a phenomenon where low doses of environmental stress lead to a favorable physiological response (e.g., increased tolerance to subsequent stress). To examine how hormesis manifests in sea anemones, we subject Nematostella vectensis to heat stress early in development and tracked growth and physiological characteristics over time. Larvae were initially primed at ambient (18°C) or elevated (24, 30, 35, and 39°C) temperatures for 1 hour, 3 days post-fertilization (DPF). They were thereafter regularly assessed for size, development, metabolic rate, protein content, and heat tolerance (lethal temperature at which 50% of larvae die [LT50]). We found that larvae primed at intermediate temperatures (24, 30, and 35°C) exhibited significantly faster growth and development and higher heat tolerance compared to larvae primed at 18°C or 39°C, characteristic of a hormetic response. There were no notable differences in total protein content or metabolic rate across temperatures or days. To investigate a candidate molecular mechanism for heat tolerance, we measured heat shock protein 70 (HSP70) at 11 DPF and found that HSP70 expression increased with priming temperature. In particular, significant upregulation of HSP70 in larvae primed at 39°C as well as the presence of doublet bands in the Western blot analysis reveal disruption of protein homeostasis due to extreme temperature priming. A weak negative correlation between HSP70 and LT50 further suggests that HSP70 alone is not responsible for controlling heat tolerance. Finally, to characterize the long-term effects of heat priming, we tracked the growth of a second cohort of heat-primed larvae over several weeks, with heat tolerance measured at 6 weeks post-fertilization. Hormetic effects on size and development persisted, whereas effects on heat tolerance were lost over time. These results collectively imply that heat tolerance as a phenotype is dynamic and influenced by genetic, environmental, and physiological factors. Overall, although extreme heat stress evidently has adverse effects on sea anemone performance, our findings strongly suggest that heat priming can boost their resilience to climate change.