Atmospheric water vapor is a major component of the global hydrological cycle, but the isotopic balance of vapor is largely unknown. Here, using models and observations, we show that the leaf water δ18O in the tropical Crassulacean acid metabolism (CAM) epiphyte Tillandsia usneoides is controlled by the δ18O of atmospheric water vapor in a predictable manner, irrespective of precipitation inputs. By taking the leaf-water-atmospheric signature as recorded in plant organic material, we have reconstructed the atmospheric water vapor δ18O signature for Miami, FL, USA between 1878 and 2005 using contemporary and herbarium specimens. T. usneoides ranges from Virginia, USA southwards through the tropics to Argentina, and the CAM epiphytic lifeform is widespread in other species. Therefore, there is significant potential for using epiphytes to reconstruct the isotope ratio of atmospheric water (both δ18O and δD) for spatial scales that span over 60° of latitude and temporal scales that cover the last century of global temperature increase.