This review evaluated the effectiveness of relaxation to lower high blood pressure. Electronic bibliographic databases and grey literature were searched to identify RCTs comparing relaxation therapies with no active treatment or sham therapy, enrolling adult participants with raised systolic blood pressure (SBP) ≥140 mmHg or diastolic blood pressure (DBP) ≥85 mmHg and follow-up ≥8 weeks. Twenty-five trials with up to 5 years’ follow-up, assessing 1198 participants, met the inclusion criteria and were meta-analysed. Overall, relaxation resulted in small, statistically significant reductions in SBP and DBP compared to the control. Substantial heterogeneity between trials (I² > 70%) was not explained by duration of follow-up, type of control, type of relaxation therapy or baseline blood pressure. The nine trials that reported blinding of outcome assessors found a non-significant net reduction in blood pressure (SBP mean difference –3.2 mmHg) associated with relaxation, as did the 15 trials comparing relaxation with sham therapy (SBP mean difference: –3.5 mmHg). Adequate randomisation was confirmed in only seven trials and concealment of allocation in only one.