Aim: Intestinal temperature telemetry systems are promising monitoring and research tools in athletes. However, the additional equipment that must be carried to continuously record temperature data limits their use to training. The purpose of this study was to assess the validity and reliability of a new gastrointestinal temperature data logging and telemetry system (e-Celsius™) during water bath experimentation and exercise trials. Materials and Methods: Temperature readings of 23 pairs of e-Celsius (T eC) and VitalSense (T VS) ingestible capsules were compared to rectal thermistor responses (T rec) at 35, 38.5 and 42°C in a water bath. Devices were also assessed in vivo during steady-state cycling (n = 11) and intermittent running (n = 11) in hot conditions. Results: The water bath experiment showed T VS and T eC under-reported T rec (P<0.001). This underestimation of T rec also occurred during both cycling (mean bias vs T VS: 0.21°C, ICC: 0.84, 95% CI: 0.66–0.91; mean bias vs. T eC: 0.44°C, ICC: 0.68, 95% CI: 0.07–0.86, P<0.05) and running trials (mean bias vs. T VS: 0.15°C, ICC: 0.92, 95% CI: 0.83–0.96; mean bias vs. T eC: 0.25, ICC: 0.86, 95% CI: 0.61–0.94, P<0.05). However, calibrating the devices attenuated this difference during cycling and eliminated it during running. During recovery following cycling exercise, T eC and T VS were significantly lower than T rec despite calibration (P<0.01). Conclusion: These results indicate that both T eC and T VS under-report T rec during steady-state and intermittent exercise in the heat, with T eC predicting T rec with the least accuracy of the telemetry devices. It is therefore recommended to calibrate these devices at multiple temperatures prior to use.