For some animals, specific microhabitats may be particularly important for certain behaviors and/or age or sex classes. Here we explore the use of previously unrecognized retreat sites (water-filled tree cavities) by Eastern Ratsnakes (Pantherophis alleghaniensis). During 4 y of radio telemetry, approximately half of the 45 ratsnakes monitored used water-filled cavities. Typically, water-filled cavities (phytotelmata) were in live Laurel Oaks (Quercus laurifolia) and Black Cherry (Prunus serotina) where limbs had broken off, internal wood had rotted, and water accumulated. Water-filled cavities were used by ratsnakes at about the same frequency as tree stumps but less frequently than snags, brushpiles, or downed logs. Snakes remained in water-filled cavities for an average of 10 d compared to only 2–4 d in other structures. Reproductive females (both pre- and post-egg laying) were four times more likely to use water-filled cavities than non-gravid or male ratsnakes, suggesting cavities are used to offset water loss associated with gestation. Ratsnakes used water-filled cavities far more in summer than spring even though thermal profiles of cavities were similar to those of other retreat structures, indicating their use was not for thermoregulation. Multiple snakes often used cavities simultaneously, suggesting that cavities are either limited or facilitate social interaction. Snakes did not use artificial water-filled cavities, suggesting that natural sites may provide snakes with some unknown benefit beyond hydration. Water-filled cavities appear to be important for ratsnakes, particularly reproductive females, and warrant further investigation.