Mastitis is a major issue in dairy cows, with subclinical mastitis (SCM) being hard to detect and potentially progressing to clinical mastitis. Antibiotic use raises concerns about resistance and milk contamination, highlighting the need for natural alternatives. Sodium alginate (SA), known for its antioxidant and immunomodulatory properties, may offer a solution, though its effects on mastitis are unclear. Intramammary infusion of 1% SA (30 mL) was tested in both healthy cows (n = 8; somatic cell count, SCC ≤ 100,000 cells/mL) and those with SCM (n = 12; SCC ≥ 200,000 cells/mL). The results showed that SA significantly increased SCC in both healthy and SCM cows, with peak levels at 48 h, returning to baseline levels thereafter. In cows with SCM, SA treatment led to a 58.3% cytological and 54.5% bacteriological cure rate after 14 days. Additionally, significant downregulation was observed in tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-2, IL-4, IL-6, and interferon (IFN)-γ. Conversely, the levels of IL-8, IL-10, and IL-12 initially increased, then declined gradually. Importantly, there were no significant effects on milk composition. These findings suggest that SA may offer an alternative to antibiotics, aiding in immune response and bacterial clearance without the risk of antibiotic residues, thus preventing SCM progression to clinical mastitis.