Recent developments of innovative anticancer therapies are based on compounds likely to stimulate the immune defense of the patients. β-(1 → 3)-Glucans are natural polysaccharides well-known for their immunostimulating properties. We report here on the synthesis of small oligo-β-(1 → 3)-glucans characterized by thioglycosidic linkages. The presence of sulfur atom(s) was not only crucial to prolong in vivo immunoactive activities in time, compared to native polysaccharides, but sulfur atoms also had a direct impact on the development of colorectal cancer stem cells. As a result, a short, pure, and structurally well-defined trisaccharidic thioglucan demonstrated similar activities compared to those of natural laminarin.