Context: It is unknown how valid esophageal, rectal, and gastrointestinal temperatures (TES, TRE, and TGI) compare after exercise-induced hyperthermia under different hydration states. Objective: To examine the differences between TES, TRE, and TGI during passive rest following exercise-induced hyperthermia under 2 different hydration states: euhydrated (EU) and hypohydrated (HY). Design: Randomized crossover design. Setting: Controlled laboratory setting. Participants: 9 recreationally active male participants (mean ± SD age 24 ± 4 y, height 177.3 ± 9.9 cm, body mass 76.7 ± 11.6 kg, body fat 14.7% ± 5.8%). Intervention: Participants completed 2 trials (EU and HY) consisting of a bout of treadmill exercise (a 10-min walk at 4.8-7.2 km/h at a 5% grade followed by a 20-min jog at 8.0-12.1 km/h at a 1% grade) in a hot environment (ambient temperature 39.3 ± 1.0°C, relative humidity 37.6% ± 6.0%, wet bulb globe temperature 31.3 ± 1.5°C) followed by passive rest. Main Outcome Measures: Root-mean-squared difference (RMSD) was used to compare the variance of temperature readings at corresponding time points for TRE vs TGI, TRE vs TES, and TGI vs TES in EU and HY. RMSD values were compared using 3-way repeated-measures ANOVA. Post hoc analysis of significant main effects was done using Tukey honestly significant difference with significance set at P < .05. Results: RMSD values (°C) for all device comparisons were significantly different in EU (TRE-TGI, 0.11 ± 0.12; TRE-TES, 1.58 ± 1.01; TGI-TES, 2.04 ± 1.19) than HY (TRE-TGI, 0.22 ± 0.28; TRE-TES, 1.27 ± 0.61; TGI-TES, 1.16 ± 0.76) (P < .01). Across the 45-min bout of passive rest, there were no differences in TRE, TGI, and TES between EU and HY trials (P = .468). Conclusions: During passive rest after exercise in the heat, TRE and TGI were in good agreement when tracking body temperature, with a better agreement appearing in those maintaining a state of euhydration versus those who became hypohydrated during exercise; however, this small difference does not appear to be of clinical significance. The large differences were observed when comparing TGI and TRE with TES.