The term “modification” often appears in physics, chemistry, biology, etc., but rarely in structural mechanics, this study proposes the concept of mechanically modified metastructures, which aims to modify conventional mechanical structures so that they can exhibit super-properties never possessed. In this study, we first introduce the coplanarly tension–torsion coupling (TTC) element into ligaments, and then analytically express the effect of the TTC component on the deformation of the general ligamentous model. Then, it is demonstrated that the TTC component can greatly tailor the mechanical properties of the prototypical structure via numerical and experimental results. The positive Poisson’s ratio can be transformed into the negative Poisson’s ratio by the mechanical modification. Moreover, it is found that the introduced TTC component can assist structures to open new bandgaps, and the location, width, and number of bandgaps can be further tuned by changing the mass and stiffness of the TTC component. This study provides a guideline for altering the mechanical properties of the structures not by changing the main geometric characteristics of the prototypical structure but only by introducing a small structural component.