Abstract: Sodium alginate (SA), a natural polysaccharide derived from brown algae, is widely recognized for its excellent biocompatibility and favorable food-processing properties. To investigate the immunomodulatory effects of low-molecular-weight sodium alginate (LSA) and its influence on gut microbiota, a cyclophosphamide-induced immunosuppressed mouse model was established to evaluate the immune-enhancing activity of LSA. The results indicated that oral administration of LSA led to a significant increase in body weight, spleen index, and thymus index, as well as an elevation in peripheral white blood cell counts. Moreover, it promoted the secretion levels of interleukin-2 (IL-2), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), and interferon-γ (IFN-γ) in both serum and spleen. In addition, LSA facilitated the proliferation of T lymphocyte subsets, improved splenic and thymic histological structures, and ameliorated cyclophosphamide-induced colonic damage. High-throughput 16S rDNA sequencing analysis revealed that LSA significantly increased gut microbiota diversity and the relative abundance of beneficial bacterial taxa, such as Bacteroides, Alistipes, and Prevotellaceae_UCG-001. Spearman correlation analysis further demonstrated positive correlations between these dominant bacteria and immune-related cytokines (IL-2, IL-6, TNF-α, and IFN-γ), while Firmicutes and Patescibacteria exhibited negative correlations with some immune-related factors. Collectively, these findings suggest that LSA may exert immune-enhancing effects through modulating the composition of gut microbiota, highlighting its potential as a functional food ingredient with immunomodulatory properties.