Aims: The long-term effect of high altitude hypoxia (HAH) exposure is a relevant problem in cardiovascular biomechanics, that has not yet been fully assessed. Regarding this, the study examined the passive viscoelastic response of the descending thoracic aorta of individuals exposed to HAH. Main methods: Through a guinea-pig animal model, subjected to either normoxic-normobaric or HAH conditions (groups N and H, respectively), biaxial tensile and uniaxial stress relaxation tests were performed on artery samples. The experimental data obtained from these biomechanical tests allowed us to characterize an anisotropic quasi-linear viscoelastic model based on the Gasser-Holzapfel-Ogden (GHO) hyperelastic material. Key findings: Among the main results, biaxial tensile tests exhibited a trend towards a higher stiffness (at high stretch levels) in the hypoxic group. Results of stress relaxation tests revealed a similar behavior between groups, at both the initial, more pronounced stress-relaxation stage, attributed to the effect of elastin fibers, and at the second stage, with a less pronounced decrease in stress due to the role of collagen fibers. Significance: Our study suggests that although HAH does not alter significantly the passive elastic and viscous properties of aortic tissue under ex-vivo conditions, there is a tendency to material stiffening to supra-physiological levels. Assessment of the biomechanical response is crucial to determine the pathophysiological effects in the cardiovascular system derived from exposure to HAH.