Characterization of viscoelastic properties of the human thigh skin can be utilized in many medical or engineering applications such as a surgical extension of the thigh skin, a tissue engineering, and a finite element modeling of thigh skin in a sitting posture. This study aims to determine the effective short- and long-term shear moduli of posterior thigh skin using ramp-relaxation test in a sitting posture. The effect of indentation location, the sitting posture, and the applied load (thigh weight) were investigated on the extracted effective shear moduli. We modeled the human skin by using the one- and two-term Prony series, and it was found that the generalized Maxwell model with two-term Prony series agreed well with experimental data. The effective shear moduli (short- and long-term) were extracted by fitting the total reaction force of the generalized Maxwell model to the experimental data using the Levenberg-Marquardt algorithm. The contour maps were used to show the spatial dependency of the effective shear moduli at the flat regions of posterior thigh skin. The contour maps of effective shear moduli show that maximum effective shear moduli locate near buttock’s center, while minimum effective shear moduli locate at the distal and medial posterior thigh. It is also found that the extracted effective short-term shear modulus varies between 3978.2 N/m2 and 13699.2 N/m2. On the other hand, the extracted effective long-term shear modulus differs between 2715.1 N/m2 and 9194.3 N/m2 for different sitting postures. Additionally, it is found that the observed increase in effective shear moduli could be attributed to the increase applied load, and leg angle.