Concrete is the most widely used engineering material. While strong in compression, concrete is weak in tension and exhibits low ductility due to its low crack growth resistance. With increasing compressive strength, concrete becomes even more brittle, hence requiring appropriate reinforcement to enhance its ductility. This paper presents a new method for increasing the ductility of ultra-high-performance concrete by reinforcing it with 3D printed polymeric lattices made of either polylactic acid (PLA) or acrylonitrile butadiene styrene (ABS). These lattice-reinforced concrete specimens were then tested in compression and four-point bending. The effect of polymeric reinforcement ratios on mechanical properties was investigated by testing two lattice configurations. The lattices were very successful in transforming the brittle ultra-high-performance concrete (UHPC) into a ductile material with strain hardening behavior; all flexural specimens revealed multiple cracking and strain hardening behavior up to peak load. Increasing the ABS reinforcing ratio from 19.2% to 33.7% resulted in a 22% reduction in average compressive strength. However, in flexure, increasing the PLA reinforcing ratio from 19.2% to 33.7% resulted in a 38% increase in average peak load. The compression results of all specimens independent of their reinforcement ratio revealed smooth softening behavior in compression. Display omitted •Ultra-high-performance concrete was reinforced with 3D-printed, polymeric lattices, resulting in greatly increased ductility.•Ductility was optimized by deliberately orienting 3D printed polymer filaments in line with the expected tensile stresses.•The ductility-enhancing mechanisms during flexure are associated with multiple cracking and tortuous crack paths•This fabrication method allows easy pouring of the mortar mixture, unlike polymer fiber-reinforced composites.•This composite production method lends itself more readily to automated manufacturing than conventional steel rebar-reinforced concrete.