Climate change is causing widespread shifts in species phenology, morphology, and distribution, with organisms moving to higher latitudes or altitudes as previously suitable areas become inhospitable. Bats are particularly vulnerable to warming due to their strong thermoregulatory constraints and temperature-dependent reproductive biology. Here, we present the results of a long-term study of Myotis daubentonii—a riparian bat species exhibiting altitudinal sexual segregation—along a river in Central Italy. Montane environments, with their compressed climatic gradients, are especially well-suited to detect such fine-scale responses. Historically, reproductive females were limited to downstream sections below 850–900 m a.s.l., where warmer conditions supported successful breeding. Our long-term data show that females have shifted their reproductive range upwards by 175 m, now regularly occupying elevations of up to approximately 1050 m a.s.l., in response to rising temperatures. We tested whether this shift represented a true expansion of the reproductive range rather than merely a foraging extension. Radio-tracking 14 reproductive females confirmed that maternity roosts have indeed been established at higher elevations. These roosts were consistently found in riparian forests, especially in rot cavities positioned high above the ground, within sparsely vegetated understories, with south-eastern exposures. Concurrently, we observed a generational increase in body size, likely driven by reduced thermoregulatory costs and improved energy allocation to offspring at higher, yet now warmer, altitudes. The observed shift, affecting only one sex and stage of life, exemplifies a subtle range expansion that might otherwise go unnoticed. Our findings emphasise the dual ecological response of M. daubentonii to climate change—both distributional and morphological—and underscore the vital role of riparian forests as ecological corridors that enable adaptive range shifts. Conservation strategies should prioritise protecting and restoring riparian habitats to bolster population resilience amid ongoing environmental change.