<p>This study investigates the applicability of micromechanical models for predicting the mechanical properties of short fiber-reinforced 3D-printed composites. Basalt fiber-reinforced poly(lactic acid) composites are analyzed by microstructural measurements to determine fiber length and fiber orientation factors. Micro-computed tomography analysis identifies a linear relationship between fiber content and fiber orientation, enabling an improved method for estimating the fiber orientation correction factor. The application of this correction allows the prediction of Young’s modulus and tensile strength while reducing the number of measurements required. The modified Halpin–Tsai model shows high accuracy, with an average error of 7% for Young’s modulus and 18% for tensile strength for fiber contents ranging from 5 to 25 w%. In addition, an anisotropy ratio was introduced as a dimensionless parameter to quantify the directional dependence of the tensile properties. Since 3D-printed fiber-reinforced composites are commonly used in load-bearing structures, understanding their anisotropic behavior is essential for optimal component design. These results contribute to improving the predictability of mechanical properties, supporting the efficient design of 3D-printed composite parts for engineering applications.</p>