The paper describes an experimental study of wind energy harvesting by self-sustained oscillations (flutter) of a flexible piezoelectric membrane fixed in a novel orientation called the “inverted flag”. We conducted parametric studies to evaluate the influence of geometrical parameters of the flag on the flapping behavior and the resulting energy output. We have demonstrated the capability for inducing aero-elastic flutter in a desired wind velocity range by simply tuning the geometrical parameters of the flag. A peak electrical power of ∼5.0mW/cm3 occurred at a wind velocity of 9m/s. Our devices showed sustained power generation (∼0.4mW/cm3) even in low-wind speed regimes (∼3.5m/s) suitable for ambient wind energy harvesting. We also conducted outdoor experiments and harvested ambient wind energy to power a temperature sensor without employing a battery for energy storage. Moreover, a self-aligning mechanism to compensate for changing wind directions was incorporated and resulted in an increase in the temperature sensor data output by more than 20 times. These findings open new opportunities for self-powered devices using ambient wind energy with fluctuating conditions and low speed regimes.