Background Since its emergence in the 1980s, acceptance and commitment therapy (ACT) has become a reputable evidence-based psychological therapy for certain disorders. Trials examining the efficacy of ACT are spread across a broad spectrum of presentations, such as chronic pain, anxiety, and depression. Nevertheless, ACT has very rarely been trialed as an intervention for suicidal ideation (SI) or deliberate self-harm (DSH). Objective The objective of this review is to assess the efficacy of ACT in reducing SI and DSH and to examine the suitability of reported SI, DSH, and other measures in determining the efficacy of ACT. Methods We systematically reviewed studies on ACT as intervention for SI and self-harm. Electronic databases, including MEDLINE, PubMed, EMBASE, PsycINFO, SCOPUS, Cochrane Central Register of Controlled Trials, and the Cochrane Database of Systematic Reviews, were searched. The reference lists of included studies and relevant systematic reviews were examined to identify additional publications. Search terms were identified with reference to the terminology used in previous review papers on ACT and suicide prevention. The study design was not restricted to randomized controlled trials. Screening was completed by 2 reviewers, and all duplicates were removed. Publications were excluded if they were not published in English, were multicomponent therapy or were not based on ACT, or lacked a validated measure or structured reporting of SI/DSH outcomes. Results After removing the duplicates, 554 articles were screened for relevance. Following the screening, 5 studies that used ACT as an intervention for suicidal or self-harming individuals were identified. The studies used diverse methodologies and included 2 case studies, 2 pre–post studies, and 1 mHealth randomized controlled trial. Conclusions The review found that ACT is effective in reducing SI in the 2 pre–post studies but not in other studies. However, given the small number and lack of methodological rigor of the studies included in this review, insufficient evidence exists for the recommendation of ACT as an intervention for SI or DSH.