Abstract: To elucidate the anisotropic mechanical behavior of 3D-printed concrete (3DPC) and quantify the effects of interlayer interface defects and pore structure, this study combines X-ray computed tomography (X-CT) scanning with grey entropy theory to systematically compare the evolution of compressive strength, splitting tensile strength, and flexural strength of cast concrete and 3DPC over curing ages of 1–28 d under the F_X, F_Y, and F_Z loading directions. X-CT-based pore-structure characterization and grey entropy relational analysis are further employed to investigate the mechanical response characteristics of 3DPC. The results show that 3DPC exhibits pronounced mechanical anisotropy. The F_Y direction provides the highest compressive strength, which is approximately 25% lower than that of cast concrete. In terms of failure modes, cast concrete predominantly exhibits overall crushing, whereas 3DPC shows interlayer delamination and layered fracture under F_X- and F_Z-direction loading, and directional splitting under F_Y-direction loading. X-CT results reveal irregular pores and elongated defects in the filament/interlayer contact regions of 3DPC specimens, and the total porosity of the scanned 3DPC specimens is higher than that of the corresponding cast specimens at the same curing age. Grey entropy relational analysis indicates that compressive strength has the highest degree of association with curing age; splitting tensile strength shows nearly equal degrees of association with curing age and the printing-path mechanism; whereas flexural strength is more strongly associated with the printing-path consistency component and the interlayer-interface normal component. The mechanical anisotropy of 3DPC is essentially governed by the competition between weak interlayer-interface defects and the directional reinforcement effects induced by printed filaments and fiber orientation. Prolonged curing improves matrix compactness and overall strength but cannot fundamentally eliminate the direction-dependent defects induced by the printing process. The findings provide a theoretical basis for printing-path design, interlayer-interface strengthening, and mechanical-performance optimization of 3DPC.