Heat therapy can significantly relieve pain in primary dysmenorrhea. A systematic review and meta-analysis included 57 randomized controlled trials with 5,359 women. Compared with no treatment, heat therapy reduced pain intensity by 1.85 cm on a 10 cm VAS scale at 3 months (25 studies, 2,393 women, 95% CI −2.29 to −1.41 cm, RD 21%) and by 3.52 cm at 24 hours (3 studies, 248 women, 95% CI −5.01 to −2.02) cm, RD 45%). It provided comparable or slightly better relief than NSAIDs at 3 months (22 studies, 1938 women, ZMD −1.10 cm, 95% CI −1.51 to −0.70 cm, RD 4%) and within 24 hours (−2.86 to −0.15 cm, 2 studies, 167 women, RD 16%). Heat therapy probably reduces the risk of adverse effects compared to NSAIDs (8 studies, 728 women, RR 0.30, 95% CI 0.15–0.59). It is effective for both prevention and acute treatment with a good safety profile.