The small-strain stiffness modulus G0 is not directly applicable to evaluate deformation in most practical engineering problems, and therefore, nonlinear soil behavior curves have been developed in terms of Young modulus E or shear modulus G. These curves were successfully obtained for sedimentary soils, while for residual soils, the available data from intertional community is still scarce. Residual soils are considered structured soils and often classified as problematic soils since they do not fit into the behavior of remolded or unstructured soils. In fact, the role of bonding and fabric affects both the strength and the stiffness of the soil. This article illustrates the use of the seismic dilatometer test (SDMT) for the determination of in situ stiffness decay curves with strain level (G-γ curves or similar) in a granitic residual soil of the Northeastern region of Portugal (Guarda), showing its adequacy to solve these kinds of problems. In situ and laboratory tests were performed on these granitic residual soils. The adopted approach relies on the ability of SDMT to provide a small-strain modulus G0 (from the shear wave velocity Vs) and a “working strain” modulus GDMT (derived from the constrained modulus MDMT in accordance with the theory of elasticity). Thus, in situ G-γ decay curves were tentatively constructed by fitting curves through these two points using a hyperbolic model. However, it was observed that the results obtained by the hyperbolic model for the cemented soils are not satisfactory. Based on the SDMT parameters, a logistic curve was derived for the reproduction of reference stiffness curves obtained from the triaxial test results.