The main aim of this paper is to experimentally and numerically determine the mechanical properties of thermoplastics that exhibit porosity due to 3D-printing. The mechanical properties of thermoplastics (polyacrylonitrile-co-butadiene-co-styrene, polycarbonate, polyetherimide) were investigated using both tensile testing machine and finite element method. The object of the study is the standard 3D-printed specimen for tensile testing. According to the result of the experimental determination of the stress–strain curves of specimens under partial loading, full unloading and repeated loading, a gradual accumulation of plastic strain was revealed for all considered thermoplastics. A finite element model for testing standard tensile specimens has been developed and validated. As a result of validation, multilinear isotropic hardening material law was identified for all investigated thermoplastics. Measured mechanical properties of porous thermoplastics were compared with properties declared by the manufacturer for the original raw material.