Background: Population aging and immunosenescence pose immense challenges to healthcare systems by increasing susceptibility to infections. Lactiplantibacillus plantarum LP28 (L. plantarum LP28) is a strain isolated from traditional Taiwanese dried tofu with significant immunomodulatory potential. Objective: To evaluate the clinical efficacy of long-term L. plantarum LP28 supplementation in common cold prevention and its effect on systemic immune biomarkers in middle-aged and elderly individuals. Methods: In this 12-week, randomized, double-blind, placebo-controlled trial, 102 middle-aged and older adults (aged 50-75 years) were assigned to receive an oral probiotic (5 x 109 CFU L. plantarum LP28/capsule, two capsules daily; n = 51) or a placebo (n = 51). Cold incidence and symptom severity were evaluated by the Wisconsin Upper Respiratory Symptom Survey (WURSS). Immune cell populations, serum/salivary markers, and natural killer (NK) cell activity were quantified using flow cytometry, ELISAs and NK cell cytotoxicity assays, respectively. The data were analyzed using a refined per-protocol approach and outlier-removed sensitivity analyses to ensure robustness. Independent t tests, chi-square/Fisher's exact tests, and Mann-Whitney U tests were used for statistical comparisons. Results: The probiotic group exhibited a notable reduction in cold incidence (34.78% vs. 52.00% in the placebo group) and a lower cumulative symptom burden. In terms of immunomodulatory effects, L. plantarum LP28 supplementation significantly increased serum interleukin (IL)-12 levels and NK cell cytotoxicity, reinforcing innate immune defense through the activation of the IL-12/NK cell axis. This was accompanied by a significant reduction in the level of the inflammatory marker C-reactive protein (CRP). Conclusions: L. plantarum LP28 supplementation serves as an effective supportive strategy for promoting healthy aging by increasing respiratory defense resilience and mitigating systemic inflammatory stress through pleiotropic immune modulation.