Perioperative neurocognitive disorders (PND) after cardiac surgery are common complications that adversely affect postoperative recovery, functional independence, and long-term prognosis. Their pathogenesis is multifactorial, involving cerebral hypoperfusion, microembolism, CPB-related systemic inflammation, BBB disruption, oxidative stress, metabolic dysregulation, neuroimmune imbalance, and host vulnerability. This review summarizes current evidence on major biomarker categories, including neuronal and axonal injury, glial activation, BBB dysfunction, systemic and neuroinflammation, oxidative stress, neurotrophic signaling, metabolic disturbance, extracellular vesicles (EVs), microRNAs, gut-brain axis-related signals, and clinical vulnerability factors. Neurofilament light chain (NfL), glial fibrillary acidic protein (GFAP), and several inflammatory markers have the most direct clinical evidence, although none has shown sufficient standalone performance for routine use. EVs, microRNAs, metabolomic profiles, pro-resolving mediators, autonomic-neuroimmune indicators, and microbiome-derived signals remain exploratory. However, most candidate biomarkers show limited discriminatory power when used alone, and few have demonstrated incremental value beyond established perioperative risk factors, including age, frailty, preoperative cognitive impairment, hemodynamic instability, and CPB-related variables. Biomarker-informed intervention studies remain predominantly preclinical or hypothesis-generating. Future progress will require standardized outcome definitions, harmonized sampling strategies, multicenter validation, and temporally structured multimodal models tailored to specific clinical purposes, including preoperative risk stratification, early postoperative brain injury monitoring, prognostic assessment, and treatment-response evaluation.