A locking system for scaffolding structures wherein said scaffolding structures comprise modular scaffolding elements being provided with respective tubular elements telescopically coupled to each other, the locking system comprises said tubular elements, a series of through holes which are formed on said tubular elements and are aligned in pairs along an axis transversal to the respective pair of coupled tubular elements, and at least one locking pin designed to define a removable joint between at least two tubular elements of said scaffolding structure, the locking pin comprises a head and a shank, which is designed to be inserted through said holes formed in two of said tubular elements telescopically coupled in order to axially and angularly lock the tubular elements together, the shank has a free end portion which, when the locking pin is inserted in the holes, protrudes outside the tubular elements on the opposite side with respect to the head, the locking pin also comprises at least one sleeve which surrounds an intermediate portion of the shank, has a bulging shape with respect to the shank and is elastically deformable in a substantially radial direction with respect to the shank so as to deform during the insertion of the shank through the holes and then expands and return to the undeformed configuration when it is arranged inside the tubular elements so as to prevent the locking pin from accidentally being disengaged out of said holes the system being characterized in that, the tubular elements are coupled telescopically to each other and in that said sleeve comprises on the axially opposite ends two collars, which delimit therebetween at least one intermediate tubular portion in the shape of a barrel; a series of circumferential through slits are formed on said intermediate tubular portion and extend approximately parallel to a longitudinal axis of the shank and are angularly spaced from each other said intermediate tubular portion is structured so that in the maximum expansion condition, corresponding to the undeformed configuration, its maximum diameter is greater than the diameter of the through holes 5 in order to elastically counteract the disengagement of the locking pin from the holes. wherein said scaffolding structures comprise modular scaffolding elements being provided with respective tubular elements telescopically coupled to each other, the locking system comprises said tubular elements, a series of through holes which are formed on said tubular elements and are aligned in pairs along an axis transversal to the respective pair of coupled tubular elements, and at least one locking pin designed to define a removable joint between at least two tubular elements of said scaffolding structure, the locking pin comprises a head and a shank, which is designed to be inserted through said holes formed in two of said tubular elements telescopically coupled in order to axially and angularly lock the tubular elements together, the shank has a free end portion which, when the locking pin is inserted in the holes, protrudes outside the tubular elements on the opposite side with respect to the head, the locking pin also comprises at least one sleeve which surrounds an intermediate portion of the shank, has a bulging shape with respect to the shank and is elastically deformable in a substantially radial direction with respect to the shank so as to deform during the insertion of the shank through the holes and then expands and return to the undeformed configuration when it is arranged inside the tubular elements so as to prevent the locking pin from accidentally being disengaged out of said holes the system being characterized in that, the tubular elements are coupled telescopically to each other and in that said sleeve comprises on the axially opposite ends two collars, which delimit therebetween at least one intermediate tubular portion in the shape of a barrel; a series of circumferential through slits are formed on said intermediate tubular portion and extend approximately parallel to a longitudinal axis of the shank and are angularly spaced from each other said intermediate tubular portion is structured so that in the maximum expansion condition, corresponding to the undeformed configuration, its maximum diameter is greater than the diameter of the through holes 5 in order to elastically counteract the disengagement of the locking pin from the holes.