Polyuria represents a significant challenge in patients with acute brain injury (ABI), with far-reaching consequences for fluid balance, electrolyte homeostasis, and cerebral perfusion. Traditionally viewed as a secondary complication, emerging evidence suggests that polyuria may also serve as an indicator of renal dysfunction, highlighting a complex interplay between cerebral and renal pathophysiology. Notably, polyuria can be an expression of a renal dysfunction and co-exist with augmented renal clearance, a phenomenon of increased renal excretion of circulating solutes. Despite its clinical relevance, polyuria lacks a universally accepted definition and standardized diagnostic approach, leading to uncertainties in prevalence estimation and management strategies. This scoping review explores the multifactorial nature of polyuria in ABI, emphasizing its potential role as a marker of renal dysfunction and its impact on patient outcomes. We propose a comprehensive diagnostic algorithm supported by biochemical markers, neuroimaging, and clinical assessment to guide early recognition and intervention. Additionally, the mnemonic “VIDEOS” is introduced as a practical tool for clinicians to streamline polyuria evaluation. By addressing both cerebral and renal dimensions of polyuria, this review aims to enhance clinical understanding and promote integrated management strategies to improve patient care and outcomes in ABI.