The shear-wave velocity structures of the crust and uppermost mantle of northern Vietnam were analyzed using the receiver function (RF) method at 25 broadband stations to investigate the regional crustal structure and its tectonic evolution. In this study, we presented a new crustal shear-wave velocity structure of northern Vietnam determined through RF analysis. Our results revealed significant variations in crustal thickness and deep crustal velocities across the study area. Along the Red River shear zone (RRSZ), the patterns of the crustal structure were distinct on both sides; they were simple and complex, respectively, in the blocks on northeast and southwest. A low-velocity zone (LVZ) was widely observed in the northwestern corner of the study area, and significant lateral variations in the thickness and strength of the crustal structure were observed from north to south. This LVZ was distributed as a thick and deep zone in the north and became thinner and shallower in the central region; the LVZ finally disappeared in the south. Two end members of the origin of the LVZ were proposed. The LVZ can be considered a weak crustal layer that escaped from the southeastern margin of the Tibetan Plateau, or it may have been formed from a paleo-subducted slab beneath it because of an onsite mantle heat source. The existence of this LVZ suggests that the movement of the RRSZ is possibly concentrated above the LVZ and that extension to the upper mantle is not necessary in the present stage. The above tectonic regime supports the possibility that the RRSZ is a strike-slip fault with a feature restricted in the crust.