The gut microbiota plays a critical role in various immunological and physiological functions in humans and animals. The intestinal bacterium Bacteroides fragilis has long been recognized as an opportunistic pathogen, especially for the formation of abdominal abscess. However, recent studies indicate that this bacterium may also help to maintain intestinal homeostasis. In this review, we focus on the role of capsular polysaccharides produced by this bacterium and describe the functions of these molecules on host immunity. The studies reviewed here indicate that capsular polysaccharides from B. fragilis have dual immunomodulatory effects on the host, depending on the immune cells and physiological context involved.