Bacterial cellulose (BC), an eco-friendly nanomaterial produced by microbial fermentation, has emerged as a versatile platform for advanced applications in electronics, optoelectronics, photonics, and energy technologies. Its unique three-dimensional nanofibrillar network, combined with remarkable mechanical strength, high crystallinity, and tailorable surface chemistry, enables exceptional performance in multifunctional devices. In electronics, BC serves as an excellent substrate for flexible and transparent conductive films, stretchable sensors, and wearable electronics, due to its superior mechanical flexibility and compatibility with various conductive materials. For optoelectronic applications, BC-based composites exhibit tunable optical properties and efficient charge transport characteristics, making them promising candidates for organic photovoltaics and light-emitting devices. In photonics, BC’s nanostructured matrix facilitates precise control over light-matter interactions, enabling applications in structural coloration, optical sensors, and biodegradable photonic crystals. The energy sector benefits from BC’s hierarchical porosity and ion-conducting properties, where it demonstrates outstanding performance as separators in batteries (e.g., Li-ion, Zn-ion), electrodes in supercapacitors, and proton-exchange membranes in fuel cells. Particularly noteworthy is BC’s role in emerging energy harvesting systems, including triboelectric nanogenerators and osmotic power generators, where its piezoelectric properties and ion selectivity enable efficient energy conversion. With ongoing advances in functionalization strategies and scalable production, BC is poised to play a pivotal role in developing next-generation sustainable and high-performance devices across these interdisciplinary fields.