BackgroundThe prevalence of nonsuicidal self-injury (NSSI) is a major concern in public health. Two main factors (individual and environmental) cause NSSI. Studies addressing NSSI often consider it as an emotion regulation strategy. Studying NSSI within the framework of attachment theory is reasonable since the capacities to regulate emotion come into existence in the framework of attachment in the first periods of a child’s growth. Primary studies addressing this topic are not frequent, and no systematic review has been conducted.ObjectiveThis systematic review and meta-analysis protocol aims to investigate the relationship between NSSI and attachment style and finding its relationship based on study design, study type, different types of attachments, and gender.MethodsAll studies on the relationship between NSSI and attachment will be included in this systematic review. We will include observational studies (cross-sectional, cohort, and case control) through searches in electronic databases via PubMed, CINAHL, Embase, Web of Science, ProQuest, Scopus, PsycINFO, and Google Scholar as complementary search. Qualitative studies, case studies, case series, and letters to the editor will be excluded. There will be no language limitation. Moreover, there will be no limitations regarding the study participants’ age, gender, nationality, sexual orientation, and psychological problems. Funnel plots were examined if 10 or more studies are included, and the Begg and Egger statistical tests were used to assess the risk of bias. All similar data will be combined through the “metan” command by Stata statistical package (StataCorp). A fixed-effects or random-effects model, considering methodological similarities or differences, will be selected to determine a combination model.ResultsWe will summarize the selection of the eligible studies using a flowchart. The results will be presented in a table of evidence. The results of the meta-analysis will be depicted using diagrams and tables.ConclusionsIt seems necessary to carry out such systematic and comprehensive meta-analysis to present a summary of the published articles in terms of the relationship between NSSI and attachment. The results from this review will be used to improve our knowledge of the role of the upbringing of children and NSSI behavior and will help design appropriate interventions to address NSSI.Trial RegistrationPROSPERO CRD42021226455; https://tinyurl.com/yc77wny8International Registered Report Identifier (IRRID)PRR1-10.2196/40808