Ion gels have the potential to be used in a broad range of applications, such as in carbon dioxide separation membranes and soft electronics. However, their low mechanical strength limits their practical applications. In this study, we developed double-network (DN) ion gels composed of TEMPO-oxidized cellulose nanofibers with hydrophobic groups (TOCNF) and cross-linked poly[1-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide] (PC_2im-TFSI) networks. The mechanical strength of the gel increased as the amount of TOCNF in the gels increased up to 6 wt%. Moreover, the fracture energy of the DN ion gels with 6 wt% TOCNF was found to be 19 times higher than that of the PC_2im-TFSI single network (SN) ion gels. Cyclic stress–strain measurements of the DN gels showed that the loading energy on the gels dissipates owing to the destruction of the physically cross-linked TOCNF network in the gels. The DN ion gels also exhibited a high decomposition temperature of approximately 400 °C because of the thermal stability of all components. Additionally, the fracture energy of the TOCNF/poly(ionic liquid) (PIL) DN ion gel was two times higher than that of the silica nanoparticles/PIL DN ion gel developed in our previous study [Watanabe et al., Soft Matter, 2020, 16, 1572–1581]. This suggests that fiber-shaped nanomaterials are more effective than spherical nanomaterials in enhancing the mechanical properties of ion gels. These results show that TOCNF can be used to toughen PIL-based ion gels and hence broaden their applications.