AbstractHybrid aerial underwater vehicles (HAUVs) that are capable of autonomously operating in both the aerial and underwater environments with repeated rapid transitions have received great interest in recent years. Current HAUV prototypes cannot fully cope with the unstructured, harsh, and hazardous ocean environments considering the high‐pressure and the strong hydrodynamic perturbations. This work reports on the HAUV, Nezha‐IV, with all basic functions validated in a real ocean environment. Nezha‐IV has four aerial propellers and eight underwater thrusters. It is not only capable of maneuvering delicately during flight and underwater but can also perform dynamic transitions under rough sea conditions. We present the design, construction, control, and oceanic testing of Nezha‐IV and demonstrate reliability and robustness, especially during the transition phases. Performance estimation and results gathered from tests in the South China Sea show that Nezha‐IV can operate at various depths ranging from 0 to 50 m at a velocity of 1 kt for 22 min and hover in the air for 15 min or cruise at a velocity of 10 m/s for 7.2 km in 12 min. This project contributes to the field of HAUV with (i) a HAUV capable of dynamic locomotion with high stability in both media and during transitions; (ii) identification of challenges during water–air transition in harsh sea conditions, and a reliable transition strategy that helps the development of fully autonomous HAUVs; (iii) a modular, foldable, and impact‐resistant HAUV structure; (iv) vehicle's capability proved by extensive tests in dynamic ocean environment.