A new biopolymer obtained from onion pulp (Allium cepa L.) was employed to produce a sustainable substrate for flexible organic light-emitting diodes (FOLEDs). Indium tin oxide (ITO) and SiO₂ thin films were deposited by rf-magnetron sputtering onto these biosubstrates to obtain flexible, transparent, and conductive anodes, on top of which FOLEDs were produced. This new biomaterial exhibits an optical transparency of 63% at 550 nm. ITO films were optimized by varying rf power during deposition onto the biopolymers, and their electrical properties are comparable to the those of ITO grown on top of rigid substrates: a carrier concentration of −3.63 × 10²¹ cm^–3 and carrier mobility of 7.72 cm² V^–1 s^–1 for the optimized film. Consequently, the sheet resistance and resistivity of this ITO film were 8.92 Ω sq^–1 and 2.23 × 10^–4 Ω cm, respectively, hence allowing the production of FOLEDs. The A. cepa L. based FOLED was fabricated using CuPc, β-NPB, and Alq₃ as organic layers, and it exhibited a maximum luminance of about 2062 cd m^–2 at 16.6 V. The current efficiency reached a maximum value of 2.1 cd A^–1 at 85.3 mA cm^–2. The obtained results suggest the possibility to use these substrates for innovative biocompatible applications in optoelectronics, such as photodynamic therapy.