With the ongoing advancement of 3D printing technology in the construction sector, the requirement for materials with optimized fresh and mechanical properties is becoming more critical. Natural fiber-reinforced mortars can be an effective solution by enhancing material’s printability. This work evaluates the effect of cellulose microfibers (CF) on fresh and hardened properties of 3D printable mortars. A reference mortar was prepared with 0–1 mm sand and 0.4 water-to-cement ratio, and three volumetric fractions of fibers were used: 2.5, 7.5, and 12.5% by mortar volume. Fresh-state properties were studied using flow table and fresh compression tests. Physical and mechanical tests were also conducted to assess the effect of fibers on the hardened state properties. A robotic 3D printer was used to assess mortar printability. Fresh state results demonstrated that CF significantly increased rheological properties over time, enhancing thixotropic behavior. 3D printing tests showed that CF improved buildability, although limited extrudability and shortened the printing open time. Additionally, CF increased 28-days mechanical strength of mortars; however, they also enlarged capillary water absorption, which could impact durability. The results underscore the potential of CF to modulate fresh and hardened mechanical properties of 3D printable mortars for construction applications.