Background <p>As 700,000 people per year commit suicide worldwide, improvements in suicide prevention and the prediction of suicide attempts or suicide in clinical care are mandatory. This systematic review aims to examine heart rate (HR) and heart rate variability (HRV) as risk factors for the development of suicidal thoughts and behaviour in clinical and non-clinical populations.</p> Methods and analysis <p>The systematic review will be conducted in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA). We will retrieve relevant literatures across the following databases: PubMed, PsycINFO, Cochrane Library and Web of Science. Additionally, we will manually search the reference lists of all relevant articles. If studies are published or translated in English and measure HRV or HR and suicidal thoughts or behaviour, they will be included. Two reviewers will independently complete the article selection, data extraction and risk of bias ratings. A third reviewer will resolve disagreements. Tabular and narrative synthesis will be done accordingly and a risk of bias assessment will be conducted by the QUIPS (Quality In Prognosis) tool.</p> Discussion <p>This systematic review will present evidence from which conclusions can be made regarding the relationship between HRV and HR and suicidal thoughts and behaviour. By identifying risk factors associated with suicidality and differentiating between factors for suicidal thoughts versus behaviour, this review will potentially contribute theoretically to our understanding of the causal factors involved in influencing increasing levels of suicidality.</p> Ethics and dissemination <p>This is a systematic review of published literature and thereby ethical approval was not sought. Results will be disseminated through conferences and publications in relevant peer-reviewed journals.</p> Strength and limitations of this study <p>The results will have important implications for risk prediction for suicidal thoughts and behaviour. The protocol follows the PRISMA guidelines (Page et al.,BMJ 372:n71,2021). Thus, two reviewers will perform data extraction and risk of bias evaluation separately. We will exclude non-English translated or published studies, which might be a limitation and bias against non-English-speaking countries.</p> Systematic review registration <p>PROSPERO registration number: CRD42023460068</p>