The white spot syndrome virus (WSSV), Vibrio parahaemolyticus and V. alginolyticus are serious epidemic pathogen affecting the cultured Portunus trituberculatus and resulted in severe economic losses to local farmers. The immune and antioxidant systems are believed to be closely involved in host responses to pathogens in aerobic animals, including crustaceans. In order to explore such host–pathogen interactions in the early stage of infection in P. trituberculatus, the mRNA transcript levels of six key immune-related genes (proPO, α2M, crustin, lysozyme, NOS, and NOX) and three key antioxidant-related genes (CuZnSOD, CAT, and GPx) and their corresponding enzymatic activity were investigated in response to challenge with the three pathogens. A decrease in the expression of the proPO, crustin, and lysozyme was observed, which may reflect the immunosuppressive mechanism of the pathogens against the host. Moreover, an increase was observed in the α2M expression with time, which indicated that the pathogens could affect proteinase cascade reactions associated with the proPO system by disturbing the balance between serine proteinases and their inhibitors. Moreover, WSSV, V. parahaemolyticus and V. alginolyticus induced to increase the transcription and enzyme activities of NOS and NOX. Additionally, significant variations in the expression of the anitioxidant-related genes CuZnSOD, CAT, and GPx and their enzyme activities implied that these enzymes played a critical role in the immune response against the pathogens. The present findings indicate that the immune parameters analyzed here may be markers of the physiological status of this species after bacterial or viral infections.