Aiming to overcome both the structural and commercial limitations of flexible thermoelectric power generators, an efficient room‐temperature aqueous selenization reaction that can be completed in air within less than 1 min, to directly fabricate thin β‐Ag2Se films consisting of perfectly crystalline and large columnar grains with both in‐plane randomness and out‐of‐plane [201] preferred orientation, is designed. A high power factor (PF) of 2590 ± 414 µW m−1 K−2 and a figure‐of‐merit (zT) of 1.2 ± 0.42 are obtained from a sample with a thickness of ≈1 µm. The maximum output power density of the best 4‐leg thermoelectric generator sample reach 27.6 ± 1.95 and 124 ± 8.78 W m−2 at room temperature with 30 and 60 K temperature differences, respectively, which may be useful in future flexible thermoelectric devices.