Cancer is a major global health concern, and the increasing number of cases underscores the need for innovative therapeutic strategies. Immunotherapy, which harnesses the immune system to target cancer cells, has shown significant progress in cancer treatment. One promising approach involves modulating the gut microbiota using prebiotics like inulin. Inulin has demonstrated antitumor effects by promoting the growth of beneficial bacteria in the gut, enhancing immune cell infiltration in tumors, and stimulating interferon-γ production, ultimately inhibiting tumor growth. However, inulin supplementation may have negative effects in certain contexts, such as intestinal pathologies like colitis and colorectal cancer. The underlying mechanisms of inulin's antitumor effects involve the production of short-chain fatty acids, including hydrogen, during fermentation. Hydrogen, with its antioxidant, anti-inflammatory properties, and potential immune-boosting effects, has shown promise in cancer treatment. Further research is needed to fully understand the molecular and cellular mechanisms behind the immune stimulation and antitumor effects of inulin and hydrogen, paving the way for novel cancer therapies.