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Severe Mental Illness (SMI) represents a significant public health challenge associated with long-term disability, premature mortality, and high social and health care costs. Assertive Community Treatment (ACT) is an intensive community-based model for people with SMI who have difficulty engaging with conventional services and are at high risk of repeated hospitalizations. Over time, this model has been adapted to meet diverse needs, contexts, and populations. To map and synthesize evidence on ACT and ACT-derived outreach models, including adaptations, fidelity-related characteristics, comparator conditions, and outcomes. Following Joanna Briggs Institute methodology and PRISMA-ScR guidelines, we searched Medline (PubMed), Web of Science, Cochrane Library, Scopus, CINAHL, PsycInfo, ProQuest, IBECS, and LILACS without year restrictions. Two reviewers independently assessed eligibility, with disagreements resolved by a third reviewer. The protocol was registered in the Open Science Framework (DOI: https://doi.org/10.17605/OSF.IO/DPNYT ). We included 129 studies (1990–2023), comprising randomized controlled trials, descriptive studies, and quasi-experimental designs. Evidence came predominantly from high-income countries and included diverse populations. Alongside classic ACT/PACT, adaptations included Flexible ACT, early psychosis community intervention, forensic and homelessness-focused models, and integrated dual-diagnosis approaches. Implementation features, fidelity reporting, comparator services, and outcome measurement varied, particularly for recovery-oriented outcomes. ACT and ACT-derived outreach services represent a continuum of intensive community mental health care. Reduced inpatient service use and improved engagement were commonly reported, whereas quality of life, social inclusion, vocational outcomes, and recovery were more variable. Better reporting of intervention characteristics, fidelity indicators, and comparator conditions is needed.