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Abstract Background The optimal psychotherapy duration for mental health disorders is unclear. Our aim was to assess the beneficial and harmful effects of shorter- versus longer-term psychotherapy for adult mental health disorders. Method We searched relevant databases and websites for published and unpublished randomised clinical trials assessing different durations of the same psychotherapy type before June 27, 2022. Our methodology was based on Cochrane and an eight-step procedure. Primary outcomes were quality of life, serious adverse events, and symptom severity. Secondary outcomes were suicide or suicide-attempts, self-harm, and level of functioning. Results We included 19 trials randomising 3,447 participants. All trials were at high risk of bias. Three single trials met the required information size needed to confirm or reject realistic intervention effects. One single trial showed no evidence of a difference between 6 versus 12 months dialectical behavioral therapy for borderline personality when assessing quality of life, symptom severity, and level of functioning. One single trial showed evidence of a beneficial effect of adding booster sessions to 8 and 12 weeks of internet-based cognitive behavioral therapy for depression and anxiety when assessing symptom severity and level of functioning. One single trial showed no evidence of a difference between 20 weeks versus 3 years of psychodynamic psychotherapy for mood- or anxiety disorders when assessing symptom severity and level of functioning. It was only possible to conduct two pre-planned meta-analyses. Meta-analysis showed no evidence of a difference between shorter- and longer-term cognitive behavioural therapy for anxiety disorders on anxiety symptoms at end of treatment (SMD: 0.08; 95% CI: -0.47 to 0.63; p = 0.77; I2 = 73%; four trials; very low certainty). Meta-analysis showed no evidence of a difference between shorter and longer-term psychodynamic psychotherapy for mood- and anxiety disorders on level of functioning (SMD 0.16; 95% CI -0.08 to 0.40; p = 0.20; I2 = 21%; two trials; very low certainty). Conclusions The evidence for shorter versus longer-term psychotherapy for adult mental health disorders is currently unclear. We only identified 19 randomised clinical trials. More trials at low risk of bias and at low risk of random errors assessing participants at different levels of psychopathological severity are urgently needed. Systematic review registration PROSPERO CRD42019128535.