Thermoplastic polyurethane (TPU) is one of elastomers featuring high elasticity and resistance to mechanical loading. It combines elastomeric properties with the capability to be processed using methods typically used for thermoplastic materials. This makes TPU an attractive alternative to conventional injection moulding materials. TPU has a wide range of applications, including automotive, medical, and footwear industries, as well as automation systems. The objective of this study was to determine the ultimate tensile stress value and strain behaviour of TPU under uniaxial tensile loading. The specimens of 20%, 30%, 50% and 90% infill density were prepared as per ISO 37:2017, using the Fused Deposition Modelling (FDM) technique. The specimens did not have upper and lower solid layers. The following mechanical properties were determined based on the output of tensile tests conducted with an optical extensometer, using the chosen methods and the obtained stress-strain curves: tensile strength, elongation at break, elasticity modulus, and yield strength. The results presented in this paper allowed determination of ultimate tensile stress vs. infill density relationship.