A toy building element (2, 2', 2") for a modular building game; the toy building element (2, 2', 2") comprising one or more hollow coupling studs for coupling the toy building element (2, 2', 2") to another toy building element (2, 2', 2") in a releasable manner; each coupling stud extending along a respective longitudinal axis and internally defining an axial through cavity configured to receive, in a removable manner, a locking stud apt to lock the toy building element (2, 2', 2") to another toy building element (2, 2', 2") coupled thereto; each coupling stud comprising a coupling part and a radially non-deformable receiving part for receiving the coupling part of a coupling stud of another toy building element (2, 2', 2"); the coupling part comprising at least a radially flexible portion for engaging/disengaging the receiving part of a coupling stud of another toy building element (2, 2', 2"); wherein the coupling part of a coupling stud of one toy building element (2, 2', 2") is configured to engage the receiving part of a coupling stud of another toy building element (2, 2', 2") without cooperating with the radially flexible portion of the coupling part of the coupling stud of the other toy building element (2, 2', 2"), so that such radially flexible portion remains radially deformable so as to allow the coupling part of the one toy building element (2, 2', 2") to disengage from the receiving part of the other toy building element (2, 2', 2"); the radially flexible portion (11, 11") being configured to cooperate with one said locking stud engaging the cavity so that its radial bending is limited by the locking stud so as to prevent the coupling part from disengaging from the receiving part of a coupling stud of another toy building element (2, 2', 2"). the toy building element (2, 2', 2") comprising one or more hollow coupling studs for coupling the toy building element (2, 2', 2") to another toy building element (2, 2', 2") in a releasable manner; each coupling stud extending along a respective longitudinal axis and internally defining an axial through cavity configured to receive, in a removable manner, a locking stud apt to lock the toy building element (2, 2', 2") to another toy building element (2, 2', 2") coupled thereto; each coupling stud comprising a coupling part and a radially non-deformable receiving part for receiving the coupling part of a coupling stud of another toy building element (2, 2', 2"); the coupling part comprising at least a radially flexible portion for engaging/disengaging the receiving part of a coupling stud of another toy building element (2, 2', 2"); wherein the coupling part of a coupling stud of one toy building element (2, 2', 2") is configured to engage the receiving part of a coupling stud of another toy building element (2, 2', 2") without cooperating with the radially flexible portion of the coupling part of the coupling stud of the other toy building element (2, 2', 2"), so that such radially flexible portion remains radially deformable so as to allow the coupling part of the one toy building element (2, 2', 2") to disengage from the receiving part of the other toy building element (2, 2', 2"); the radially flexible portion (11, 11") being configured to cooperate with one said locking stud engaging the cavity so that its radial bending is limited by the locking stud so as to prevent the coupling part from disengaging from the receiving part of a coupling stud of another toy building element (2, 2', 2").