Abstract: Toy building block comprising a hollow three-dimensional structure with a plurality of faces and provided on at least one of said faces with engagement means able to co-operate with complementary engagement means of another block. The three-dimensional structure is formed by a pliable element with a perimeter whose shape corresponds to the planar development of the block. The pliable element is provided with hinges between adjacent portions forming the faces of the three-dimensional structure. Snap-in fastening means are provided for fastening each of said faces to an adjacent face of the three-dimensional structure.