Diamine-functionalized cellulose nanocrystals (DamCNCs) prepared by periodate oxidation and reductive amination were used as a scaffold for the in-situ oxidative polymerization of polyaniline (PANI). Unlike unmodified CNCs, the hybrid PANI/DamCNC particles formed a stable colloidal suspension over a large pH domain, PANI particles having nucleated and grown from the nanocrystals. The hybridization of PANI was validated by transmission electron microscopy images, along with Fourier-transform infrared, Raman, and UV–Vis spectra. By taking advantage of the colloidal stability of the PANI/DamCNC nanohybrids, flexible and freestanding thin nanopapers were prepared, with different contents in PANI/DamCNCs, using cationic or anionic cellulose nanofibrils (CCNFs or ACNFs, respectively). Conductive nanopapers were obtained by mixing CCNFs with PANI/DamCNCs at a content higher than 20 wt%, exhibiting high flexibility, mechanical integrity, transparency, and stability over time. This work proposes a simple process to fabricate a novel class of biobased conductive hybrid nanofiller based on PANI grafted on DamCNCs, stable in water, that may be used as an additive in nanocellulose suspensions to produce green flexible thin nanopapers, with potential use in different types of electronic devices such as electronic displays, smart packaging, or paper-based supercapacitors for energy storage.