Recently, numerous attempts have been made to evaluate the potential of chitosan as an adjuvant; however, few have explored the mechanism underlying the adjuvant activity of chitosan. Here, we used hepatitis B surface antigen (HBsAg) as a model antigen for the formulation of an intranasal chitosan-based vaccine against hepatitis B. In vitro and in vivo assays were conducted to evaluate its efficacy and explore the possible underlying mechanism of action. Our results showed that HBsAg was efficiently encapsulated within chitosan particles, and that the strong adhesive effect of the positive charges on the particle surface prolonged the residence in the nasal cavity. The insolubility of chitosan particles in physiological pH enabled the formation of a sustained-release depot in extracellular fluids, whereas their solubility in aqueous acids facilitated the escape of the encapsulated antigen from lysosomes, thereby enabling cross-presentation. Chitosan was able to open epithelial-cell tight junctions, thus allowing the entry of free antigens. In addition, chitosan enhanced the uptake of antigens by dendritic cells and promoted their maturation. In vivo results showed that intranasal delivery of chitosan particles induced significantly higher levels of cellular and mucosal immunity than did injected alum-based vaccine. These findings are highly useful for the rational development of new and improved vaccines.