In the demand for sustainable and environmentally friendly energy solutions, the development of lead-free piezoelectric materials has attracted significant attention over the past decades. Lead-free (K, Na)NbO3 (KNN) is a promising candidate for substituting traditional lead-based piezoelectric materials due to its excellent physical properties. In this work, we demonstrate the synthesis and characterization of flexible KNN-based piezoelectric thin films. The 0.5 mol% Mn-doped KNN (KNMN) films are fabricated on flexible metal foil substrates using a chemical solution deposition method. The crystal structure and morphological properties of KNMN films are investigated by various experimental analyses. Moreover, the device performance characteristics of flexible KNMN thin films are examined through a bending test. An output voltage of ~0.12 V is generated under the mechanical bending stress. Our study highlights the viability of KNN-based thin films as a sustainable and eco-friendly solution for next-generation energy harvesting technologies.