An assembly for joining together modules of a modular wind turbine blade. According to one embodiment the assembly is formed by a plurality of bolts that are secured between pairs of first and second inserts respectively housed in the composite material of first and second modules. According to one embodiment each assembly is formed by first and second lateral caps, upper wedge and a lower wedges located between the first and second lateral caps, and transverse screws joining the upper and lower wedges that together are formed around the corresponding bolt. When a force F1 is applied to the wedges, the lateral caps respond with forces F2 that urge the first and second modules away from one another to longitudinally stress the bolt.