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Abstract Threat of predation often requires animals to seek refuge in unusual or suboptimal habitats. While aquatic refugia are thought to be used by many terrestrial animal species, there are challenges associated with aquatic refugia that could preclude their actual usefulness, particularly to air‐breathing ectotherms. Consequently, observations that can shed light on the evolution of antipredator strategies that cross the air–water interface are particularly valuable. “Semi‐aquatic” spiders in the family Trechaleidae have not been documented using diving as part of their antipredator behavior and, indeed, they are rarely known to submerge themselves at all, as their aquatic foraging occurs from a terrestrial location. The lack of diving behavior is hypothesized to be due to the high potential costs (e.g., thermal and respiratory) of submergence. Despite this, we document the prolonged underwater refuge use (>30 min) ofTrechalea extensafollowing pursuit by a perceived threat (humans). We also identify some morphological adaptations that may potentially help counterbalance these costs and permit underwater antipredator behavior, specifically the ability of cuticular hairs to provide a hydrophobic surface, allowing the formation of a plastron that, in turn, may facilitate respiratory function and reduce heat loss. These observations expand our overall understanding of habitat use in this genus and, more generally, antipredator adaptations of spiders.