The authors recently developed a 3D-printable ultra-high performance fiber-reinforced concrete (3DP-UHPFRC) for additive construction of structural members with significantly reduced reliance on steel bars. This study investigates the rheological behavior of the developed 3DP-UHPFRC. The effects of two major factors affecting the performance of 3DP-UHPFRC, namely steel fiber volume (0, 1%, and 2%) and nano-clay (NC) content (0, 0.1%, and 0.2% by binder mass) on workability, static yield stress, dynamic yield stress, and apparent viscosity were determined. Test results showed that the inclusion of steel fibers and NC reduced the workability and led to a significant increase in the static yield stress, dynamic yield stress, and apparent viscosity. However, the effect of NC content on the rheological properties became negligible in the mixtures made with 2% fiber content. Similarly, the effect of steel fiber volume became negligible in the mixtures made with 0.2% NC. In addition, the influence of changes in rheology due to the addition of steel fiber and NC on the extrudability and buildability of the mixtures were investigated by 3D-printing of 500 mm high hollow columns with three different print speeds. The results showed all mixtures exhibited satisfactory extrudability (i.e., no blockage of extruder or tearing of filaments was observed). In addition, the buildability of the mixtures increased as the steel fiber and NC contents increased.