TEMPO-oxidized cellulose nanofibrils (TOCNF) and oxidized carbon nanotubes (CNT) were used as humidity-responsive films and evaluated using electroacoustic admittance (quartz crystal microbalance with impedance monitoring, QCM-I) and electrical resistivity. Water uptake and swelling phenomena were investigated in a range of relative humidity (% RH) between 30 and 60% and temperatures between 25 and 50 °C. The presence of CNT endowed fibril networks with high water accessibility, enabling fast and sensitive response to changes in humidity, with mass gains of up to 20%. The TOCNF-based sensors became viscoelastic upon water uptake, as quantified by the Martin-Granstaff model. Sensing elements were supported on glass and paper substrates and confirmed a wide window of operation in terms of cyclic % RH, bending, adhesion, and durability. The electrical resistance of the supported films increased by ∼15% with changes in % RH from 20 to 60%. The proposed system offers a great potential to monitor changes in smart packaging.