AbstractTemperature is a key environmental factor influencing animal behavior, including foraging, predator avoidance, and habitat use. Intertidal fishes already experience extreme thermal variability and may be especially vulnerable to further environmental change; therefore, understanding how their behavior responds to temperature fluctuations is essential. This study investigates how chronic exposure to two ecologically relevant temperatures (10 °C and 20 °C) shapes the behavior of the intertidal common triplefin, Forsterygion lapillum. We use a series of behavioral assays to assess activity, emergence, feeding, evasive responses, and aggression. During the 6-week acclimation period, fish in the 20 °C treatment consumed more food and spent less time hidden within refuges. Fish acclimated to 20 °C also exhibited greater overall activity, increased aggression, and a shorter latency to initiate movement. However, evasive responses to a simulated bird strike did not differ significantly between treatments. These findings demonstrate that warmer conditions can substantially alter the behavior of intertidal triplefins. While elevated activity and aggression may bolster foraging success and territorial defense, they also bring higher energetic demands and increased risk of predator exposure. These trade-offs may ultimately influence individual survival, reproductive success, and population-level outcomes. In the context of ongoing climate change, our findings underscore the need to understand species-specific behavioral responses to warming. Even subtle behavioral shifts could cascade through intertidal communities, affecting ecological stability.