Agar, a heterogeneous polymer of galactose, is the main component of the cell wall of marine red algae. It is well established as a safe, non-digestible carbohydrate in oriental countries. Neoagarooligosaccharides (NAOs) prepared by hydrolyzing agar by microbial β-agarase have been reported to show safety. However, their immunological effects have not been reported yet. Thus, the objective of this study was to investigate immune enhancing effects of neoagarooligosaccharides (NAOs) from marine red algae Gelidium elegans in mice by performing ex vivo experiments. Six-week-old mice were fed ad libitum. NAOs were orally administrated at three different concentrations (100, 500, and 2,500 mg/kg B.W./day) twice a week for four weeks. The group fed with NAOs at 2,500 mg/kg showed the highest proliferation of splenocytes and production levels of cytokines (IL-1β, IL-6, TNF-α) in the ex vivo experiment. In conclusion, NAOs can enhance immune function, increase proliferation of splenocytes, and increase cytokine production by activating macrophages in mice.