Crab aquaculture has gained interest all over the world and is a delicacy in many places. Despite the recent progress made by crab aquaculture, diseases are of great concern in the crab industry. Until recently, the major tool for preventing and treating diseases in crab and other aquatic species was antibiotics; however, the use of antibiotics has its own problems calling for alternatives of which probiotics, prebiotics, and synbiotics have proven to be a suitable alternative. Treating bacterial diseases with antibiotics as prophylactic agents is proven not to be sustainable and ineffective. This is because pathogens are able to develop resistance. The purpose of this review was to look at the use of probiotics, prebiotics, and synbiotics in the crab aquaculture industry. From the available literature, it is evident that these immunostimulants have a major role to play in crab culture. The review shows that probiotics, prebiotics, and synbiotics improve the immune response of crabs. It also improves the microbiota of the gut, which leads to better digestion, absorption, and use of diets. It is evident that much has not been done with respect to the molecular mechanism regulating the effects these natural ingredients have on the immune system of crabs. It will, therefore, be prudent that further studies be carried out on how these ingredients affect growth, survival, and immune systems of crab, especially synbiotics which has not been given attention.