Formwork for a hanging beam protruding below a floor slab, whereby the formwork comprises a plurality of lateral panels (3, 4) and lower panels, wherein each panel has an inner face (21, 31, 41) forming a respective face of the beam, and wherein, in an operating formwork position, the lateral panels (3, 4) of each side are arranged vertically and parallel to each other, separated from one another by the lower panels arranged horizontally, such that the longitudinal edges (23, 24) of the lower panels are in flat contact with the inner faces (31, 41) of the lateral panels (3, 4) through mutual affixing means (52, 53, 54, 55, 56, 57), wherein the affixing of the lower panels with respect to the lateral panels (3, 4) is height-adjustable in order to adapt the formwork to hanging beams with different heights, and in that each lower panel is composed of two semi-panels (201, 202), the first semi-panel being longer than the second semi-panel, wherein each semi-panel (201; 202) comprises an inner forming face (211; 212), an outer face (221; 222), two longitudinal edges (231, 241; 232, 242), an end transverse edge (251; 262), and a connecting transverse edge (271; 272) that connects to the connecting transverse edge (272; 271) of the other semi-panel (202; 201), with which it forms one same lower panel, wherein the interior of a semi-panel is made up of the outward-facing open space delimited by its outer face and by its longitudinal edges and transverse edges, all of which is configured in such a way that in the coupling position of the two semi-panels (201; 202) of one same lower panel, the inner faces (211; 212) and outer faces (221; 222) of the two semi-panels are flush, constituting the inner face and the outer face of the lower panel, respectively, the longitudinal edges (231, 241; 232, 242) of the two semi-panels being arranged in a coplanar manner constituting the longitudinal edges (23, 24) of the lower panel, and the end transverse edge of the first semi-panel and the end transverse edge of the second semi-panel constituting the two transverse edges (25, 26) of the lower panel, wherein the two semi-panels (201, 202) can be removably coupled to one another to constitute the lower panel through removable attaching means configured for their connecting transverse edges (271, 272) to be attached by flat contact, characterized in that the connecting transverse edge of the first semi-panel forms an obtuse angle with its inner face and the connecting transverse edge of the second semi-panel forms an acute angle supplementary to the aforementioned obtuse angle; and in that each second semi-panel is coupled to a shoring post for supporting the lower panels, enabling the decoupling of the first semi-panels from the rest of the formwork, after previously removing the lateral panels (3, 4), leaving the second semi-panels supported by the corresponding shoring posts; and in that the second semi-panel of each lower panel is provided with housing means to removably couple, in the second semi-panel, the end of a bolt or peg which can be inserted by its other end into the upper end of the shoring post or support, said housing means being coupled below the outer face and arranged in a rotary manner around a transverse axis parallel to the direction towards which the end transverse edge of the second semi-panel extends, configured in such a way that the shoring post or support inserted in the housing means through the bolt or peg is able to adopt a position wherein it ranges from at least an acute angle up to a right angle with respect to the outer face of the second semi-panel.