Modular formwork for making reinforced concrete ribbed floors, suited to be laid on a temporary bearing structure consisting of parallel beams, wherein it comprises: • a plurality of central modular elements suited to rest on said temporary bearing structure, defining the nodes of a grid with square or rectangular meshes, each one of said central modular elements comprising a body substantially parallelepiped in shape, with a plane perimetral part and a central raised part substantially parallelepiped in shape, and wherein the upper surface of said perimetral part of the central element comprises at least one seat or lowered part distributed on its perimeter, • a plurality of modular elements in the form of joists, each suited to rest and be constrained with its short opposite terminal sides between two of said central modular elements aligned with each other, said seat or lowered part of the central element suited to allow the short opposite terminal sides of said joist to rest and be inserted therein, said joists being arranged in such a way as to define the lines of said grid, each one of said joists comprising a body substantially elongated and parallelepiped in shape, having two opposite long sides substantially plane and a central raised part substantially parallelepiped in shape, occupying the whole length of said joist itself, and wherein the two short opposite terminal sides of said joist have a lower edge shaped in such a way as to rest on and be coupled with said perimetral part of said central elements aligned with said joist, so that the surfaces of the two short opposite sides of said joist face each other and are in contact with corresponding lateral surfaces (A22, A23) of the raised parts of said central elements; • a plurality of dome-shaped elements, each one suited to rest on and be constrained with the edges to said joists and/or to said central modular elements, so as to cover each one of said meshes, and wherein each one of said central elements is suited to allow maximum four of said joists to rest thereon, while each one of said dome-shaped elements is suited to rest with said edges on the long sides of four of said joists arranged in such a way as to form a square or rectangular mesh of said grid, and wherein the upper surface of said edges is at the same height as the upper surface (B21, A21) of said joists and/or said central elements; and wherein the upper surfaces of said joists, said edges and the lateral surfaces (C3, C4) of said dome-shaped elements define the formwork bottom surfaces for casting the floor ribs characterized in that said central elements, said joist and said dome-shaped elements are made of rigid plastic material and are monolithic. • a plurality of central modular elements suited to rest on said temporary bearing structure, defining the nodes of a grid with square or rectangular meshes, each one of said central modular elements comprising a body substantially parallelepiped in shape, with a plane perimetral part and a central raised part substantially parallelepiped in shape, and wherein the upper surface of said perimetral part of the central element comprises at least one seat or lowered part distributed on its perimeter, • a plurality of modular elements in the form of joists, each suited to rest and be constrained with its short opposite terminal sides between two of said central modular elements aligned with each other, said seat or lowered part of the central element suited to allow the short opposite terminal sides of said joist to rest and be inserted therein, said joists being arranged in such a way as to define the lines of said grid, each one of said joists comprising a body substantially elongated and parallelepiped in shape, having two opposite long sides substantially plane and a central raised part substantially parallelepiped in shape, occupying the whole length of said joist itself, and wherein the two short opposite terminal sides of said joist have a lower edge shaped in such a way as to rest on and be coupled with said perimetral part of said central elements aligned with said joist, so that the surfaces of the two short opposite sides of said joist face each other and are in contact with corresponding lateral surfaces (A22, A23) of the raised parts of said central elements; • a plurality of dome-shaped elements, each one suited to rest on and be constrained with the edges to said joists and/or to said central modular elements, so as to cover each one of said meshes,