Additive manufacturing has created a paradigm shift in materials design and innovation, providing avenues and opportunities for geometric design freedom and customizations. Here, we report a microarchitected gyroid lattice liquid–liquid compact heat exchanger realized via stereolithography additive manufacturing as a single ready-to-use unit. This lightweight (∼240 kg/m3) compact heat exchanger (with conjoined headers), with an engineered porosity of 80% and a separating wall thickness of 300 μm, has a surface to volume ratio of 670 m2/m3. X-ray computed tomography imaging confirms a defect-free 3D printed heat exchanger. The thermo-hydraulic characteristics were experimentally measured using water as the working fluid. The measurements indicate that the heat exchanger evinces an overall heat transfer coefficient of 120-160W/m2K for hot fluid Reynolds number Reh in the range of 10-40. Additionally, finite element analysis was conducted to evaluate the thermo-hydraulic characteristics of the gyroid lattice heat exchanger. The experimental results show -a 55% increase in exchanger effectiveness for the additively manufactured gyroid lattice heat exchanger in comparison to a thermodynamically equivalent, most-efficient, counter-flow heat exchanger at one tenth of its size. The superiority of our architected heat exchanger to extant work is also demonstrated.