WHAT IS KNOWN ON THE SUBJECT?: Nursing skill mix research has primarily focused on medical and surgical patients. The association between nursing skill mix and clinical outcomes for psychiatric inpatients has not been adequately explored. No systematic reviews have been reported that have examined the association between the ratio of mental health to registered nurses and patient outcomes. WHAT THIS PAPER ADDS TO EXISTING KNOWLEDGE?: Our protocol sets out, in detail, the methods we intend to follow to ensure that this review is conducted and reported to the highest possible standards. WHAT ARE THE IMPLICATIONS FOR PRACTICE?: The review will summarize evidence about safe staffing in mental health inpatient units internationally. The review will inform health policy and planning of health services. ABSTRACT: Aim To assess the association between mental health nurse to comprehensive/general nurse ratio and relapse-determined by hospital admission-in adult psychiatric inpatients. Background information There is considerable variation in psychiatric inpatient nursing skill mix internationally. In most countries, wards are typically staffed by a mix of mental health and comprehensive/general nurses. To date, there have been no systematic reviews that have examined the association between the ratio of mental health to registered nurses and patient outcomes. Method We will systematically search five databases. Risk of bias will be determined using the Effective Public Health Practice Project Quality Assessment Tool. A meta-analysis will be undertaken if there are at least two similar studies. Results For included studies, we will produce table summarizing risk of bias and extracted data. If we are able to undertake a meta-analysis, this will be reported as a forest plot. Discussion This paper sets out a detailed plan for a systematic review of mental health nurse skill mix research. Implications for practice High-quality evidence about the association between nursing skill mix and patient outcomes is important to inform the planning of mental health services internationally.