Ganoderma formosanum (GF) is a medicinal mushroom endemic to Taiwan. Previous research established the optimal culture conditions to produce exopolysaccharide rich in β-glucan (GF-EPS) from submerged fermentation of GF. The present study investigated the antitumor effects of GF-EPS in a Lewis lung carcinoma cell (LLC1) tumor-bearing mice model. In the preventive model, GF-EPS was orally administered to mice before LLC1 injection. In the therapeutic model, GF-EPS oral administration was initiated five days after tumor cell injection. The tumor size and body weight of the mice were recorded. After sacrifice, the lymphocyte subpopulation was analyzed using flow cytometry. Spleen tissues were used to analyze cytokine mRNA expression. The results showed that GF-EPS (80 mg/kg) effectively suppressed LLC1 tumor growth in both the preventive and therapeutic models. GF-EPS administration increased the proportion of natural killer cells in the spleen and activated gene expression of several cytokines. Our results provide evidence that GF-EPS promotes tumor inhibition through immunomodulation in tumor-bearing mice.