Primary sclerosing cholangitis (PSC) is a chronic cholestatic liver disease characterized by progressive inflammation and fibrosis of intra- and extrahepatic bile ducts, ultimately leading to cirrhosis and an increased risk of cholangiocarcinoma. Increasing evidence suggests that immune dysregulation and gut–liver axis disturbance play central roles in PSC pathogenesis. This review aims to summarize current evidence regarding the role of vitamin D in PSC and its interaction with gut–liver axis homeostasis. A narrative review of current literature concerning PSC pathophysiology, vitamin D biology, immune modulation, epithelial barrier integrity, and microbiota interactions was performed using recent experimental and clinical studies. Vitamin D exerts pleiotropic immunomodulatory effects and contributes to the maintenance of epithelial barrier function and intestinal microbiota balance. Vitamin D deficiency is highly prevalent in PSC patients and is associated with cholestasis, malabsorption, and disease severity. Current evidence suggests that vitamin D may function as a regulatory “buffer” of gut–liver axis homeostasis by attenuating chronic inflammatory signaling and preserving epithelial integrity. Although observational studies indicate an association between low vitamin D levels and more severe PSC phenotypes, causality remains unproven. Further prospective and interventional studies are necessary to determine whether vitamin D may serve as a therapeutic target or primarily reflects disease severity in PSC.