This paper presents the design and implementation of a miniature underwater angle of attack (AOA) sensor, enabling real-time measurement of AOA relative to the incoming flow. To the best of the author’s knowledge, previous literature has not reported such kind of sensors for AOA measurement. Based on the Hall effect, a separated structure between a rotor with a vane and a circuit module is developed to improve the dynamic response of the sensor. Further, the utilization of an aluminum alloys hull, a 3D-printed resinous vane and ceramic bearings endow the sensor with remarkable performance while keep it a tiny volume simultaneously. In addition, waterproofness and anti-corrosion are also taken into account. Finally, the angle data acquired from the AOA sensor attached to the caudal fin of a free-swimming multi-joint robotic fish, is compared with the referenced angle obtained off-line through a global visual motion measurement system. Experiment results demonstrate the built sensor entirely meets the requirements of accurate and realtime control, which is beneficial in improving flow control and hydrodynamic optimization.