Elasmobranch fishes use electroreception to detect electric fields in the environment, particularly minute bioelectric fields of potential prey. A single family of obligate freshwater stingrays, Potamotrygonidae, endemic to the Amazon River, demonstrates morphological adaptations of their electrosensory system due to characteristics of a high impedance freshwater environment. Little work has investigated whether the reduced morphology translates to reduced sensitivity because of the electrical properties of freshwater, or because of a marine-tuned sensory system attempting to function in freshwater. The objective of the present study was to measure electric potential from prey of Potamotrygon motoro and replicate the measurements in a behavioural assay to quantify P. motoro electrosensitivity. Median orientation distance to prey-simulating electric fields was 2.73 cm, and the median voltage gradient detected was 0.20 mV cm–1. This sensitivity is greatly reduced compared with marine batoids. A euryhaline species with marine-type ampullary morphology was previously tested in freshwater and demonstrated reduced sensitivity compared with when it was tested in seawater (0.2 µV cm–1v. 0.6 nV cm–1). When the data were adjusted with a modified ideal dipole equation, sensitivity was comparable to P. motoro. This suggests that the conductivity of the medium, more so than ampullary morphology, dictates the sensitivity of elasmobranch electroreception.