Abstract Underwater 3D printing of ultra-high-performance concrete (UHPC) is one promising solution for rapid construction and repair of marine infrastructure. Compared with conventional concrete, UHPC offers superior durability, reduced permeability, and an extremely cohesive matrix. These qualities are significant to minimize washout and ensure structural integrity when printing underwater. While all the studies have been reported on traditional printable concrete mixes under submerged environments, the application of UHPC in underwater 3D printing remains largely unexplored. Furthermore, the temperature-dependent rheological behavior of cementitious materials significantly influences their extrudability, printability, and buildability under submerged printing. This research investigates the influence of underwater low-temperature conditions—particularly between 4 to 20°C—on the printability of UHPC by analyzing the relationship between pre-printing rheological properties and printing performance. UHPC mixtures are optimized for each target temperature for adequate buildability and extrudability. Viscosity and yield stress are quantified with a rheometer before printing and correlated with observed print behavior. To evaluate the feasibility of in-situ marine mixing, both tap water and seawater are independently utilized as mixing water. After determining workable printable mixtures, cubic samples are cast underwater for mechanical performance evaluation at each temperature. Compressive strength test, bi-surface shear test for interlayer bonding, and indirect splitting tensile test are performed with these cubic specimens. To simulate realistic field conditions, all printing operations are performed in seawater. This research provides new insights into the correlation between rheological properties and mechanical behavior in cold underwater environments, facilitating the development of more robust and field-adaptable underwater 3D printing processes.