The overall objective of this work was to study the influence of high-density polyethylene (HDPE) concentration on the rheological properties and microstructure of HDPE-modified binders prepared in a rotor-stator mixing device. With this aim, viscous and linear viscoelastic measurements, as well as morphological analysis and modulated differential scanning calorimetry tests, were performed on the modified binders. From the experimental results obtained, it can be concluded that the addition of HDPE to bitumen enhances the mechanical properties of the binders, especially in the high-temperature region, where pavements can be submitted to permanent deformation. The addition of 3% HDPE to bitumen is able to readily modify the structure of the binder, by forming a gel-type polymer structure dispersed within the bituminous matrix, providing it with enhanced elastic properties.