Background: Three-dimensional (3D) printing has emerged as a valuable tool in dentistry for producing provisional restorations with high precision and reduced costs. However, the limited mechanical strength of temporary 3D-printed resins remains a clinical concern. This in vitro study aimed to enhance the mechanical properties of a 3D-printed temporary resin by incorporating functionalized niobium (Nb) nanoparticles and to compare its performance with a conventional resin composite and a permanent 3D-printed resin. Methods: Six groups were evaluated: bisacrylic resin (Protemp), resin composite (Z350), temporary 3D resin (Temp 3D), permanent 3D resin (Perm 3D), Temp 3D + 0.05% Nb, and Temp 3D + 0.1% Nb. Niobium oxyhydroxide nanoparticles were synthesized using a hydrothermal method, silanized, and incorporated into the Temp 3D at 0.05% and 0.1% by weight. The tested variables included flexural strength (FS), elastic modulus (EM), surface hardness (SH), and color stability (ΔE). Results: The Z350 resin showed the best mechanical results. The addition of 0.1% Nb nanoparticles significantly improved the FS, EM, and SH of the Temp 3D, reaching values comparable to the Perm 3D (p > 0.05). Color stability remained unaffected across all groups. Conclusions: These findings suggest that Nb reinforcement at a low concentration is a promising strategy for improving the performance of 3D-printed temporary restorations.