<p>Fused-filament fabrication (FFF) has become one of the most widely used additive manufacturing (AM) methods across different industries, leading to the widespread development of diverse FFF systems and materials. However, despite its large industrial penetration, rapid prototyping still remains as its main application because the resultant mechanical properties of FFF parts are still uncertain and dependent on the material, system, and process parameters used. Although several research works have focused on understanding the structural behavior of FFF components, they cannot be well compared since they have been generated under particular conditions in terms of material, system, and process parameters. This paper aims to investigate the effect of the material and infill density on the tensile properties of FFF parts using a universalized approach that allows a proper comparison among different FFF materials and systems. This approach is based on measuring the real infill density and normalizing the tensile properties of the FFF test specimens. A comprehensive experimentation is conducted on FFF tensile specimens made of different materials and infill density values. For each specimen, the real infill density is measured, and a tensile test is conducted to determine its ultimate tensile strength (<i>UTS</i>) and elastic modulus (<i>E</i>). The results have shown that the normalized tensile properties and the real infill density have the same tendency and a nonlinear relationship for all the FFF materials and systems analyzed.</p>