Cellulose and its derivatives are attractive for the development of electronic devices because they are naturally abundant and biodegradable. Ethyl cellulose (EtC) is particularly promising as a substrate material for flexible electronics because it is waterproof with low air permeability and high transparency. In this study, adaptable EtC-Ag nanowire (AgNW) composite-based transparent electrodes (TEs) are developed for various electronic devices. Fabrication of EtC-AgNW composites is clean and safe without using toxic chemicals, resulting in highly transparent and conductive electrodes. With the assistance of functional tapes, EtC-AgNW composites are successfully used as attachable flexible transparent interconnects, and sensing electrodes for transparent electrophysiological sensors. In addition, a semitransparent perovskite solar cell is demonstrated by using the EtC-AgNW composites as a top electrode, which shows a high power conversion efficiency of 7.03%.