Zwitterionic hydrogels have received great attention due to their excellent nonfouling and biocompatible properties, but they suffer from weak mechanical strength in the saline environments important for biomedical and engineering applications due to the “anti‐polyelectrolyte” effect. Conventional strategies to introduce hydrophobic or non‐zwitterionic components to increase mechanical strength compromise their nonfouling properties. Here, a highly effective strategy is reported to achieve both high mechanical strength and excellent nonfouling properties by constructing a pure zwitterionic triple‐network (ZTN) hydrogel. The strong electrostatic interaction and network entanglement within the triple‐network structure can effectively dissipate energy to toughen the hydrogel and achieve high strength, toughness, and stiffness in saline environments (compressive fracture stress 18.2 ± 1.4 MPa, toughness 1.62 ± 0.03 MJ m–3, and modulus 0.66 ± 0.03 MPa in seawater environments). Moreover, the ZTN hydrogel is shown to strongly resist the attachment of proteins, bacteria, and cells. The results provide a fundamental understanding to guide the design of tough nonfouling zwitterionic hydrogels for a broad range of applications. An effective strategy is developed to achieve both high mechanical strength and excellent nonfouling properties in saline environments, important for biomedical and engineering applications, by constructing a pure zwitterionic triple‐network hydrogel. Strong chain entanglement and electrostatic interactions provide effective energy dissipation to improve the mechanical strength and toughness of the hydrogel.