Humidity sensing plays an important role in diverse applications in the fields of agriculture, healthcare, skincare, and environmental monitoring. With the advent of the "trillion sensor" society, there is a growing demand for humidity sensors that can be made from abundant and renewable resources. Cellulose, the most abundant and renewable bioresource on Earth, acts as a humidity-sensing material because of its hydrophilic nature. However, non-renewable noble metal electrodes are still indispensable because good electrical conductivity and moisture stability are necessary for reliably detecting electrical signals from cellulose-based humidity-sensing materials. Herein, we demonstrate the direct CO2-laser writing of electrodes onto TEMPO-oxidized cellulose paper to realize an all-cellulose-derived humidity sensor. The TEMPO-oxidized cellulose paper with sodium carboxylate groups provides a satisfactory humidity-sensing performance and is converted to conductive and moisture-stable electrodes directly via laser-induced carbonization. The resulting all-cellulose-derived humidity sensor demonstrates high sensitivity and linearity over a wide range of relative humidity (11-98%), thereby providing broad applicability. The sensor can be used for the positional localization of moist objects and for monitoring plant transpiration and human-body sweating. These results open the door for renewable and sustainable humidity sensors.