Background <p>Mental health disorders pose a major public health challenge worldwide, with one in eight individuals affected, particularly by anxiety and depression. In Nigeria, mental health issues are similarly prevalent, yet access to mental healthcare remains limited. Recent policy initiatives have increased governmental interest, but mental health research in Nigeria remains limited and fragmented.</p> Methods <p>This protocol outlines a systematic review and meta-analysis methodology to quantify the prevalence of mental health disorders in Nigeria. The review follows the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P) and Joanna Briggs Institute (JBI) guidelines and includes cross-sectional studies, surveys, and cohort studies providing prevalence data on mental disorders diagnosed by healthcare professionals or identified via validated screening tools in the country. Data sources include PubMed/Medline, EMBASE, and African Journals Online (AJOL), from inception up to April 2024. Study selection, data extraction, and quality assessment will be conducted independently by multiple reviewers. A random-effects meta-analysis model will be used to synthesize data, with sensitivity and subgroup analyses to explore heterogeneity.</p> Discussion <p>This systematic review and meta-analysis will provide an estimate of the prevalence of mental health disorders in Nigeria, informing future public health policy and resource allocation. These findings will contribute significantly to understanding mental health in Nigeria and support the development of effective strategies and policies.</p> Systematic review registration <p>PROSPERO CRD42024559090.</p>