The aim of the present study was to assess the effect of cold water immersion (CWI) on post-exercise parasympathetic reactivation. Ten male cyclists (age: 29 ± 6 y) performed two repeated supramaximal cycling exercises (SE₁ and SE₂) interspersed with a 20-min passive recovery period, during which they were randomly assigned to either 5 min of CWI in 14°C, or a control condition (N) where they sat in an environmental chamber (35.0 ± 0.3°C and 40.0 ± 3.0% relative humidity). Rectal temperature (T_re) and beat-to-beat heart rate (HR) were recorded continuously. The time constant of HR recovery (HRR) and a time varying vagal-related HR variability (HRV) index (rMSSD_30s) were assessed during the 6-min period immediately following exercise. Resting vagal-related HRV indices were calculated during 3-min periods 2 min before and 3 min after SE₁ and SE₂. Results showed no effect of CWI on T_re (P=0.29), SE performance (P=0.76) and HRR (P=0.61). In contrast, all vagal-related HRV indices were decreased after SE₁ (P<0.001), and tended to decrease even further after SE₂ under N condition; but not with CWI. Compared to the N condition, CWI increased HRV indices before (P<0.05) and rMSSD_30s after (P<0.05) SE₂. Our study shows that CWI can significantly restore the impaired vagal-related HRV indices observed after supramaximal exercise. CWI may serve as a simple and effective means to accelerate parasympathetic reactivation during the immediate period following supramaximal exercise.