Materials exhibiting a negative Poisson’s ratio, are typically referred to as auxetics, demonstrate exceptional mechanical properties like enhanced energy absorption, shear resistance, and indentation resilience. This paper investigates the design, fabrication as well as the mechanical characterization of 3D-printed auxetic composite materials. Advanced additive manufacturing techniques were employed to create optimized geometries with auxetic properties and improved mechanical performance. The composites were fabricated were produced from polylactic acid (PLA) strengthened with flax fibers (PFF), a sustainable and environmentally friendly material, through additive manufacturing. A variety of geometry were selected for their distinct auxetic behaviors and mechanical characteristics allowing for an in-depth investigation of their influence on composite sandwich performance. Tensile tests were performed to assess the material’s static performances. Results revealed that the unique geometry of the honeycomb material significantly influences these properties. Additionally, a numerical model (FEM) was developed, showing strong consistency with experimental results. This model highlighted the impact of the geometry of the composite on the mechanical response, offering insights into the interaction between material properties and geometry. This study lays the groundwork for the advancement of high-performing auxetic composites with potential applications in lightweight structural composite components.