Background Telemental health (delivering mental health care via video calls, telephone calls, or SMS text messages) is becoming increasingly widespread. Telemental health appears to be useful and effective in providing care to some service users in some settings, especially during an emergency restricting face-to-face contact, such as the COVID-19 pandemic. However, important limitations have been reported, and telemental health implementation risks the reinforcement of pre-existing inequalities in service provision. If it is to be widely incorporated into routine care, a clear understanding is needed of when and for whom it is an acceptable and effective approach and when face-to-face care is needed. Objective This rapid realist review aims to develop a theory about which telemental health approaches work (or do not work), for whom, in which contexts, and through what mechanisms. Methods Rapid realist reviewing involves synthesizing relevant evidence and stakeholder expertise to allow timely development of context-mechanism-outcome (CMO) configurations in areas where evidence is urgently needed to inform policy and practice. The CMO configurations encapsulate theories about what works for whom and by what mechanisms. Sources included eligible papers from 2 previous systematic reviews conducted by our team on telemental health; an updated search using the strategy from these reviews; a call for relevant evidence, including “gray literature,” to the public and key experts; and website searches of relevant voluntary and statutory organizations. CMO configurations formulated from these sources were iteratively refined, including through discussions with an expert reference group, including researchers with relevant lived experience and frontline clinicians, and consultation with experts focused on three priority groups: children and young people, users of inpatient and crisis care services, and digitally excluded groups. Results A total of 108 scientific and gray literature sources were included. From our initial CMO configurations, we derived 30 overarching CMO configurations within four domains: connecting effectively; flexibility and personalization; safety, privacy, and confidentiality; and therapeutic quality and relationship. Reports and stakeholder input emphasized the importance of personal choice, privacy and safety, and therapeutic relationships in telemental health care. The review also identified particular service users likely to be disadvantaged by telemental health implementation and a need to ensure that face-to-face care of equivalent timeliness remains available. Mechanisms underlying the successful and unsuccessful application of telemental health are discussed. Conclusions Service user choice, privacy and safety, the ability to connect effectively, and fostering strong therapeutic relationships need to be prioritized in delivering telemental health care. Guidelines and strategies coproduced with service users and frontline staff are needed to optimize telemental health implementation in real-world settings. Trial Registration International Prospective Register of Systematic Reviews (PROSPERO); CRD42021260910; https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42021260910