<p>Flat sheet membranes are widely used in early-stage membrane development; however, their integration into compact and modular systems for in vitro testing remains limited. This study reports the design, fabrication, and validation of a 3D-printed flat sheet membrane module (FSMM) compatible with extracorporeal blood circuits (ECBCs) for laboratory scale applications, enabling dynamic testing under hemodialysis-like conditions with minimal blood volume. Following a structured design process, two FSMM prototypes were developed and fabricated using additive manufacturing via material extrusion. Successive iterations focused on improving flow distribution and mechanical stability. The final module (t-FSMM) was integrated into an ECBC and validated through leak-tightness tests, experimental characterization of channel geometry and surface roughness, and hemolysis assays using diluted and whole bovine blood. Membrane performance was evaluated by monitoring transmembrane pressure, protein, and hemoglobin rejection. The t-FSMM met all predefined design requirements, exhibiting watertightness, leak-tightness, structural integrity, and stable membrane positioning under hemodialysis-like conditions. Whole blood filtration tests confirmed hemolysis levels below 2%, indicating a non-hemolytic system. This work is among the first to explore and validate additive manufacturing for the production of 3D-printed hemodialysis modules for in vitro testing, demonstrating its potential not only for fabrication but also as a flexible design tool for developing reproducible and customizable membrane modules suitable for minimal-volume blood filtration.</p>