This work presents a wearable sensor for real‐time on‐leaf monitoring of relative humidity (RH), temperature, and vapor‐pressure deficit (VPD) of plants in both controlled environments and under field conditions. This sensor is flexible and conformable to the leaf surface. By integrating a graphene‐based RH sensing element and a gold‐based thin‐film thermistor on a polyimide sheet, the sensor allows accurate and continuous determination of VPD at the leaf surface, thereby providing information on plant transpiration. A greenhouse experiment validates the ability of the sensor to continuously and simultaneously monitor both the leaf RH and temperature of maize plants over more than 2 weeks. The sensor output also demonstrates the influences of light and irrigation on maize transpiration. Uniquely, by attaching multiple sensors onto different locations of a plant, it is possible to estimate the time required for water to be transported from the roots to each of the measured leaves along the stalk, as well as longitudinally from one position on a leaf toward the leaf tip. Sensors are also deployed in crop production fields where they demonstrate the ability to detect difference in transpiration between fertilized and unfertilized maize plants.