Water shortage is the most important abiotic limitation for banana. The development of climate-smart cultivars and agronomic practices is ongoing, but the process is hindered by a lack of field-applicable drought stress evaluation methods. Earlier research indicated that carbon isotope discrimination values in bulk leaf material (d13Cbulk) could serve as a proxy for drought stress in banana. However, d13Cbulk is a time-integrated measure of stress. Information about current and short-term conditions could be useful for early stress detection or to observe diurnal variations in stress. Phloem sap contains recently assimilated carbon. Its d13C value (d13Cphloem) has successfully been used as a short-term indicator of drought stress in trees. We have developed a phloem sap sampling method for banana and evaluated it in a field experiment in Arusha, Tanzania. We compared d13Cphloem with d13Cbulk under two different watering treatments (irrigated and rainfed) and assessed diurnal variations by sampling in the morning (08:00) and at noon (12:00). Samples were collected during the dry season. Both d13Cbulk and d13Cphloem were significantly affected by the watering treatments, although the effect on d13Cphloem was greater. This indicates the more sensitive and recent nature of d13Cphloem. In addition, d13Cphloem differed throughout the day, while d13Cbulk did not. d13Cphloem was higher at noon than in the morning, likely due to the occurrence of instantaneous drought stress. This difference in d13Cphloem between morning and noon was larger in the irrigated treatment than in the rainfed treatment. Probably, the more severe stress in the rainfed treatment already led to partial stomatal closure in the morning, resulting in a higher d13Cphloem and a smaller difference with d13Cphloem at noon. Such short-term diurnal variations could not be detected with d13Cbulk. d13Cphloem can be used as an indicator for instantaneous drought stress, creating the possibility to investigate sh