The renal resistivity index is a tool to evaluate the hemodynamics through arterial resistance. Considering perfusion as one of the first aspects to be affected upon kidney injury, alterations in renal blood flow could be especially important for the early detection of kidney damage. The aim of this systematic review was to retrieve published studies on equine renal resistivity index (RI) in order to develop a standardized method of renal ultrasound examination through a transabdominal approach as well as to evaluate current reference range for renal RI value in horses. An electronic search in Science Direct, PubMed, Scopus and Web of Science databases was performed in February 2023 using the terms “RI” OR “resistivity index” OR “IP” OR “pulsatility index” AND (kidney OR renal) AND (equine OR horse) in titles, keywords and abstracts. The studies were screened based on inclusion criteria and variables of interest were extracted. To assess the methodological quality, the SYRCLES‘s risk of bias tool was used. From the search, a total of 134 studies were identified and five of them were considered eligible. The selected studies had been conducted in healthy non-sedated horses through transabdominal technique. The upper limit of normality for renal RI was 0.58 ± 0.06 for the right kidney of untrained horses, which is considerably lower than the value of 0.70 currently used for humans, cats and dogs. There were heterogenous outcomes among studies, where two out of the five demonstrated difference between RI value of left and right kidneys and one out of the five showed increased renal RI value in the elderly compared to foals and adult animals. Data of RI in horses is still scarce; a normality standard for renal RI studies in horses would greatly improve comparison and good quality results to allow clinical application.