We had isolated a high molecular weight polysaccharide fraction, designated as F3, and performed a comprehensive analysis of its immunomodulatory and adjuvant activities in vivo and in vitro. In vivo, F3-treated mice showed an increase in the number of dendritic cells as well as CD4, CD8, regulatory T, B, plasma, NK, and NKT cells in the spleen. F3 also elevated the levels of multiple cytokines and chemokines in the blood of mice. F3 displayed potent adjuvant activity for tetanus toxoid in the absence of alum and potentiated antibody responses to alum-containing tetanus toxoid in mice. In addition, F3 also boosted Th1 and Th2 response in vivo. In vitro, F3 induced the maturation of dendritic cells derived from human monocytes by upregulating CD40, CD54, CD80, CD83, CD86, and HLA-DR, enhanced mixed lymphocyte reaction, and stimulated the production of ten cytokines and six chemokines. In microarray analysis, expressions of 7688 genes were modulated in dendritic cells after treatment with F3, including cytokine and chemokine genes. These results provide F3 polysaccharide extract further insight into the mechanisms of action for these immunomodulatory and adjuvant activities of from Ganoderma lucidum and also offer preclinical evidence for its development as a vaccine adjuvant.