Background:Plant-based diets (PBDs) are characterised as healthy dietary patterns that emphasise the intake of plant foods and limit the intake of animal foods. The Mediterranean and Dietary Approaches to Stop Hypertension (DASH) diets are common examples of healthy dietary patterns that are mainly plant based. There are different dietary pattern analysis approaches and scoring systems available to construct indices that measure adherence to a dietary pattern. Nutritional epidemiology studies necessitate the use of appropriate dietary indices when investigating diet-disease associations. This protocol is for a review that will: 1) evaluate how a PBD has been defined in studies published globally; and 2) assess the methods used to construct dietary indices that measure adherence to a PBD; with a focus on studies that have assessed the association between adherence to a PBD and CVD risk. Methods:This protocol was developed according to the Preferred Reporting Items for Systematic reviews and Meta-Analysis Protocols guidelines. PubMed–Medline, Scopus and biomedical databases within EBSCOhost will be searched up to August 2021. Two reviewers will independently screen the identified records and review the eligible full texts for inclusion. Discrepancies will be resolved by consensus or through discussion with a third reviewer. Meta-analysis will be performed where possible and consistency of the findings checked through subgroup analysis. Heterogeneity across studies will be assessed and quantified, and publication bias investigated. Relevant sensitivity analyses will be performed to substantiate the robustness of the study findings. Conclusion:Currently, there is some inconsistency in defining and measuring adherence to a PBD across study populations. Moreover, a lack of global data on the association between adherence to a PBD and CVD risk. This systematic review could aid in promoting the worldwide uptake of these findings for policy and practice purposes. This research will use previously published studies; and therefore, will not require ethical approval.