As the recent trend of fabricating high-strength polymer has been gaining more attention in the market, researchers are exploring the possibility of using a 3D printing technique for the same. Vat-photopolymerization additive technology has good process capability and produces high-quality photopolymer parts with greater geometrical complexity. This paper proposes the vat-photopolymerization process to fabricate high-strength acrylate-based photopolymer composite with improved mechanical strength. The composite structures contain 1.0, 2.0, and 3.0 wt percentages of 13 nm average-sized alumina (Al2O3) nanoparticle (NP) as the reinforcement and acrylate photopolymer as matrix material. The viscosities of all the prepared suspensions were measured to determine the printing properties of the prepared nanocomposites, and these values are within the feasible printing limit. Furthermore, the depth of cure and time of exposure to UV light were optimized to achieve the desired level of printing performance. The thermogravimetric analysis (TGA) of the alumina nanocomposites reveals that the addition of alumina NP has minimal influence on the thermal stability of the photopolymer matrix. To ascertain the mechanical strength of the 3D printed products, tensile tests are performed, and it was witnessed that 1.0% weight fraction of alumina-NP loaded specimen experienced the highest tensile strength amongst all the weight fractions of alumina-NP considered, and its value increased by ∼48% when compared against the pure acrylate polymer sample. The increase in strength values for the alumina-NP loaded specimen was explained using dynamic mechanical analysis and optical and scanning electron micrographs. A constitutive elastic-viscoplastic isothermal non-linear time-dependent material model is also developed to predict the post-yield strain-softening behavior of these nanocomposites. The weight fraction and optimal weight fraction of alumina nanoparticles are correlated using semi-empirical equations of various model parameters.