A modular formwork for reinforced concrete walls of the type comprising: a pair of containment walls (2, 3) substantially plate-like and with assemblable modular structure, which are adapted to be arranged one in front of the other so as to form a gap adapted to contain the concrete; and a plurality of transversal spacers, which extend bridging the two containment walls (2, 3) and are fixed to both containment walls (2, 3) so as to connect said containment walls (2, 3) rigidly to one another; at least a first containment wall (2, 3) comprising: a series of assemblable infill boards, substantially parallelogram-shaped and with a modular structure, which are arranged one adjoined the other so as to form substantially the whole wall; and a plurality of interconnection joints, separated and distinct from the infill boards, which are distributed along the edges of the various infill boards, and are adapted to rigidly connect said infill boards to one other in pairs; said interconnection joints being adapted to firmly fit and interlock simultaneously on the edges of both the infill boards, and being shaped so as to embrace each infill board on opposite sides thereof; wherein the transversal spacers are separated and distinct from the interconnection joints, are structured so as to rigidly anchor to said interconnection joints and are adapted to connect the various interconnection joints of the first containment wall (2, 3) rigidly to the second containment wall (2, 3); said modular formwork being characterized in that each interconnection joint is divided into at least two pieces (30, 31; 40, 41; 50, 51) separated and distinct from one another, which are complementary in shape and are coupled/couplable to one another in easily detachable manner. at least a first containment wall (2, 3) comprising: a series of assemblable infill boards, substantially parallelogram-shaped and with a modular structure, which are arranged one adjoined the other so as to form substantially the whole wall; and a plurality of interconnection joints, separated and distinct from the infill boards, which are distributed along the edges of the various infill boards, and are adapted to rigidly connect said infill boards to one other in pairs; said interconnection joints being adapted to firmly fit and interlock simultaneously on the edges of both the infill boards, and being shaped so as to embrace each infill board on opposite sides thereof; which are distributed along the edges of the various infill boards, and are adapted to rigidly connect said infill boards to one other in pairs; said interconnection joints being adapted to firmly fit and interlock simultaneously on the edges of both the infill boards, and being shaped so as to embrace each infill board on opposite sides thereof; wherein the transversal spacers are separated and distinct from the interconnection joints, are structured so as to rigidly anchor to said interconnection joints and are adapted to connect the various interconnection joints of the first containment wall (2, 3) rigidly to the second containment wall (2, 3); said modular formwork being characterized in that each interconnection joint is divided into at least two pieces (30, 31; 40, 41; 50, 51) separated and distinct from one another, which are complementary in shape and are coupled/couplable to one another in easily detachable manner.