BACKGROUND AND OBJECTIVES: Acceptance and commitment therapy (ACT) enhances psychological flexibility by fostering acceptance of thoughts and emotions, promoting mindfulness practices, and encouraging engagement in value-based actions. These processes have been associated with improvements in mental health and social functioning, with accumulating evidence supporting ACT's efficacy across various psychiatric disorders. This systematic review aimed to evaluate current evidence on ACT interventions for reducing psychiatric symptoms and enhancing social functioning and interpersonal relationships in adults with psychiatric conditions. MATERIALS AND METHODS: A comprehensive search was conducted across PubMed, Web of Science, the Cochrane Library, and Embase for studies published between 2014 and 2024. The review protocol was registered on the Open Science Framework (OSF; registration ID: 2ZAGT). RESULTS: Seventeen studies met the inclusion criteria; however, the evidence base remained fragmented, with most psychiatric diagnoses represented by only one or two studies. The reviewed studies exhibited several methodological limitations, including small sample sizes, lack of randomization and blinding, high risk of bias, reliance on subjective outcome measures, and inadequately designed or absent control groups. Despite these limitations, ACT was associated with beneficial effects in conditions such as post-traumatic stress disorder (PTSD), insomnia, psychosis, and autism spectrum disorder, particularly in reducing experiential avoidance, enhancing mindfulness, and promoting long-term improvements in emotional regulation and life satisfaction. CONCLUSIONS: Due to the limited number of studies per diagnosis, significant methodological weaknesses, and the lack of high-quality controlled trials, this review cannot provide strong evidence for the efficacy of ACT in improving social functioning among adults with psychiatric disorders. The heterogeneity and overall low quality of the available literature highlight the urgent need for further large-scale well-controlled studies.