Background: Cold‐water therapy is gaining popularity as a mood‐enhancing intervention. However, immersion protocols vary, particularly in terms of temperature and duration. This study examined whether different durations of cold‐water immersion produce differential effects on mood.

Materials and Methods: One hundred and forty participants with self‐reported low mood were randomly assigned to one of four groups: a control group or immersion groups exposed to seawater at 13.6°C ± 0.3°C for 5, 10 or 20 min. Participants in the immersion groups completed the Profile of Mood States (POMS) 7 days before and immediately after immersion; control participants completed the same measures at matched intervals. Skin temperature and heart rate were recorded before and during immersion, and heart rate variability (HRV) was analysed in 10 participants from the 5‐min group.

Results: Total mood disturbance (TMD) significantly improved across cold‐water immersion groups. The greatest reduction was observed in the 20‐min group (mean change −15.9 points; 49.7 ± 18.8 to 33.8 ± 10.8, p < 0.0005), followed by the 5‐min group (−14.7 points; 47.6 ± 19.9 to 32.9 ± 13.7, p < 0.0005) and the 10‐min group (−8.8 points; 41.8 ± 15.5 to 33.0 ± 12.3, p = 0.001). No significant change in TMD was observed in the control group (−1.9 points; 41.5 ± 9.5 to 39.6 ± 6.4, p = 0.156). Immersion significantly reduced skin temperature by 10.7°C, (t(48) = 21.8, p < 0.0005) and increased heart rate by 33 bpm, (t(48) = –17.76, p < 0.0005). HRV showed reduced root mean square of successive differences (RMSSD) (48.8 to 28 ms) and total power (4197 to 1812 ms2), indicating a shift from parasympathetic to sympathetic dominance.

Conclusion: Cold‐water immersion appears to effectively improve mood, with benefits observed across all durations. A 5‐min immersion was as effective as longer exposures, offering a practical option for implementation. While safe in healthy, screened individuals under controlled conditions, caution is advised in less structured environments or with individuals with pre‐existing health conditions.