Background: Psychiatric disorders are highly prevalent and associated with great symptomatic, functional, and health economic burdens. Psychotherapy is among the recommended and used interventions for most psychiatric disorders and is becoming widely accessible in mental health systems. The effects of specific forms of psychotherapy (e.g., psychodynamic therapies, cognitive and behavioral therapies, humanistic therapies, and systemic therapies) have been assessed previously in systematic reviews, but the appropriate psychotherapy duration for psychiatric disorders has not been reviewed. The aim of this systematic review will be to synthesize the evidence of the effects of short-term compared with long-term psychotherapy for all adult psychiatric disorders. Methods/design: A comprehensive search for relevant published literature will be undertaken in Cochrane Central Register of Controlled Trials (CENTRAL), Medical Literature Analysis and Retrieval System Online (MEDLINE), Excerpta Medica database (EMBASE), Latin American and Caribbean Health Sciences Literature (LILACS), PsycINFO, Science Citation Index Expanded (SCI-EXPANDED), Social Sciences Citation Index (SSCI), Conference Proceedings Citation Index—Science (CPCI-S), and Conference Proceedings Citation Index—Social Science & Humanities (CPCI-SSH) to identify relevant trials. We will search all databases from their inception to the present. We will include randomized clinical trials comparing a short-term and a long-term version of the same psychotherapy type for adult psychiatric disorders including attention deficit hyperactivity disorder, psychotic disorders, depressive disorders, bipolar disorders, anxiety disorders, obsessive-compulsive disorder, trauma- and stressor-related disorders, eating disorders, and personality disorders (as defined by standardized diagnostic criteria). We will rely on the trialists defining their compared interventions as short term and long term (or similar terminology). Primary outcomes will be quality of life, serious adverse events, and symptom severity. Secondary outcomes will be suicide or suicide attempts, self-harm, and level of functioning. Two review authors will independently extract data and perform risk of bias assessment using the Cochrane risk of bias tool. A meta-analysis will be performed as recommended by the Cochrane Handbook for Systematic Review of Interventions, bias will be assessed with domains, and Trial Sequential Analysis will be conducted to control random errors. Certainty of the evidence will be assessed by GRADE. Discussion: As psychotherapy is among the treatments of choice for most adult psychiatric disorders, a systematic review evaluating the benefits and harms of short-term compared with long-term psychotherapy is urgently needed. It is the hope that this review will be able to inform best practice in treatment and clinical research of these highly prevalent and burdensome disorders.