This study investigated the effects of nano-calcium carbonate on cement mortar for 3D printing. Nano-calcium carbonate (NC) was mixed at varying additions, from 0% to 4% by cement weight. The test series included flowability, open time, layer deformation (10 layers), density, water absorption, compressive strength, and flexural strength. The results showed that increasing NC content reduced flowability and initial printing time, but all mix proportions had a final printing time of over 2 h. A higher NC content also decreased the width of filaments while increasing their height. Regarding both compressive and flexural strengths of the printed filament specimens improved as NC content increased to 2% due to the nanoparticles filling voids and enhancing the matrix density. In conclusion, adding an appropriate amount of NC improves the mechanical properties and printability of mortar, making it ideal for 3D printing applications.