Cellulose nanocrystals (CNCs) from cotton were functionalized in aqueous medium using methacrylic anhydride (MA) to produce methacrylated cellulose nanocrystals (mCNCs) with a degree of methacrylation (DM) up to 12.6 ± 0.50%. Dispersible as-prepared CNCs and mCNCs were then considered as reinforcing fillers for aqueous 3D-printable formulations based on methacrylated carboxymethylcellulose (mCMC). The rheological properties of such photo-cross-linkable aqueous formulations containing nonmodified CNCs or mCNCs at 0.2 or 0.5 wt% in 2 wt% mCMC were fully investigated. The influence of the presence of nanoparticles on the UV-curing kinetics and dimensions of the photo-cross-linked hydrogels was probed and 13C CP-MAS NMR spectroscopy was used to determine the maximum conversion ratio of methacrylates as well as the optimized time required for UV postcuring. The viscoelasticity of cross-linked hydrogels and swollen hydrogels was also studied. The addition of 0.5 wt% mCNC with a DM of 0.83 ± 0.040% to the formulation yielded faster cross-linking kinetics, better resolution, more robust cross-linked hydrogels, and more stable swollen hydrogels than pure mCMC materials. Additionally, the produced cryogels showed no cytotoxicity toward L929 fibroblasts. This biobased formulation could thus be considered for the 3D printing of hydrogels dedicated to biomedical purposes using vat polymerization techniques, such as stereolithography or digital light processing.