Human serum albumin (HSA) is the most abundant plasma protein and plays essential roles in regulating oncotic pressure, transporting endogenous and exogenous compounds, and maintaining redox balance. Because of its biochemical properties, HSA is widely used in clinical medicine, biotechnology, and pharmaceutical development. However, albumin is not a static molecule. It undergoes numerous post-translational modifications (PTMs), including oxidation, glycation, carbonylation, carbamylation, and aggregation, which can significantly alter its structure, ligand-binding properties, antioxidant capacity, and pharmacokinetics. Furthermore, commercial albumin products exhibit substantial heterogeneity arising from differences in source material, manufacturing processes, formulation conditions, and storage practices. This review summarizes the current understanding of PTMs affecting human serum albumin, with particular emphasis on their molecular origins and functional implications. We discuss how physiological, pathological, and manufacturing-induced modifications influence albumin activity and contribute to variability among commercial albumin products. In addition, we examine established and emerging applications of PTM-modified albumin in diagnostics, critical care, extracorporeal detoxification, drug delivery, nanomedicine, and regenerative medicine. Finally, we highlight current challenges related to product heterogeneity, quality control, regulatory assessment, and discuss future opportunities in albumin-based technologies. A deeper understanding of albumin PTMs is essential for improving critical care and developing next-generation albumin-based biomedical products.