AbstractIn this work, we presented a new thermoelectrochromic device based on the preparation of a hydrogel‐based electrolyte derived from cellulose, namely hydroxypropyl cellulose, with thermochromic properties and a good ion conduction at atmospheric conditions. With these premises, an electrochromic layer of PEDOT:PSS was developed to fabricate a multifunctional device capable of exploiting the TC properties of HCP when the critical temperature of 43°C is reached, where an increase in reflectance of 37% at 420 nm is reported and, at the same time, capable of undergoing a significant modulation of transmittance (52% at 650 nm) when a small external bias (2.5 V) is applied. To our knowledge, it is the first time that a smart window is designed embodying HPC and PEDOT:PSS in a single device, reporting a fully organic system suitable for several applications, from constructions to transportation.