Monitoring the flow velocity of an urban ventilation system has great significance for air circulation and air quality. In this study, a self‐powered flow velocity sensor based on hybrid piezo‐triboelectric nanogenerator (P‐TENG) is proposed. The flow velocity is detected by a triboelectric nanogenerator (TENG) working in freestanding triboelectric‐layer mode. A polyvinylidene difluoride (PVDF) fixed on the galloping beam converts the kinetic energy of the moving air into electricity. Both the TENG and PVDF are driven by the galloping vibration of a bluff body arisen from the air flow of ventilation. The structure of the P‐TENG is provided and a prototype is fabricated. The experiments indicate that the PVDF can generate 2.4 µW of energy across an external resistance of 60 MΩ. Flow velocity ranging from 4 to 10 m s−1 can be well detected by the proposed nanogenerator. Moreover, the P‐TENG is applicable to a critical environment with humidity up to 75%. Demonstration is carried out on a wind tunnel, illustrating good reliability as the frequency remains stable for the whole duration. Compared with the commercial anemometer, the error rate is under 1% after calibration. The proposed P‐TENG promises in low cost, self‐powered flow velocity monitoring in urban ventilation systems.