A stretching device for tensioning a roller curtain, the roller curtain comprising: - a flexible curtain; - a winding/unwinding roller having a support surface and rotatably mounted about a longitudinal rotation axis, said winding/unwinding roller being configured to rotate about the above mentioned longitudinal rotation axis in a first direction of rotation to cause said flexible curtain to be unwound from said support surface up to obtain an unwound configuration, or in a second direction of rotation, opposite to the first, in order to cause the flexible curtain to be wound about said support surface up to obtain a wound configuration, said flexible curtain having a first extremity portion fixed to said support surface and a second extremity portion opposite to the first extremity portion; - an actuation device arranged to cause said winding/unwinding roller to rotate in said first, or in said second, direction of rotation about said longitudinal rotation axis; said stretching device being arranged to move from a rest configuration, to a stretched configuration, said stretching device being configured to exert, in said stretched configuration, a determined stretching force, substantially horizontal, on said flexible curtain, when said flexible curtain is arranged in said unwound configuration, said stretching device comprising: - at least a hollow containment body defining a housing and having an inlet mouth to reach said housing, said hollow containment body being positionable laterally to said flexible curtain; - at least a fixed body housed, in use, in said housing; - at least a movable body configured to slide transversally to said flexible curtain and housed within said housing, said movable body being arranged to engage, in use, a lateral portion of said flexible curtain positioned through said inlet mouth; said stretching device being characterised in that it further comprises: - at least a tensioning cable (45,45a,45b) having a first end (46, 46a, 46b) fixed to a support (20, 20a, 20b) rotatably mounted about a rotation axis (120, 120a, 120b) and a second end (47,47a,47b) fixed to said respective fixed body (41,41a,41b), or fixed to said respective movable body (42,42a,42b) of said tensioning device, said tensioning cable (45, 45a, 45b) being engaged to said fixed body and to said movable body in at least a respective engagement point (Pf, Pm), said, or each, engagement point (Pf,Pm) being interposed between said first end (46,46a,46b) and said second end (47,47a,47b), said support (20, 20a, 20b) being configured to rotate about said rotation axis (120, 120a, 120b) in a first direction of rotation to subject said tensioning cable (45, 45a, 45b) to a tractive force such that it causes said movable body to move from a position distant from said fixed body, to a position close to said fixed body, in order to exert said determined substantially horizontal stretching force on said flexible curtain, which is, therefore, arranged in a laterally stretched configuration. - a flexible curtain; - a winding/unwinding roller having a support surface and rotatably mounted about a longitudinal rotation axis, said winding/unwinding roller being configured to rotate about the above mentioned longitudinal rotation axis in a first direction of rotation to cause said flexible curtain to be unwound from said support surface up to obtain an unwound configuration, or in a second direction of rotation, opposite to the first, in order to cause the flexible curtain to be wound about said support surface up to obtain a wound configuration, said flexible curtain having a first extremity portion fixed to said support surface and a second extremity portion opposite to the first extremity portion; - an actuation device arranged to cause said winding/unwinding roller to rotate in said first, or in said second, direction of rotation about said longitudinal rotation axis; - at least a hollow containment body defining a housing and having an inlet mouth to reach said housing, said hollow containment body being positionable laterally to said flexible curtain; - at least a fixed body housed, in use, in said housing; - at least a movable body configured to slide transversally to said flexible curtain and housed within said housing, said movable body being arranged to engage, in use, a lateral portion of said flexible curtain positioned through said inlet mouth; - at least a tensioning cable (45,45a,45b) having a first end (46, 46a, 46b) fixed to a support (20, 20a, 20b) rotatably mounted about a rotation axis (120, 120a, 120b) and a second end (47,47a,47b) fixed to said respective fixed body (41,41a,41b), or fixed to said respective movable body (42,42a,42b) of said tensioning device, said tensioning cable (45, 45a, 45b) being engaged to said fixed body and to said movable body in at least a respective engagement point (Pf, Pm), said, or each, engagement point (Pf,Pm) being interposed between said first end (46,46a,46b) and said second end (47,47a,47b), said support (20, 20a, 20b) being configured to rotate about said rotation axis (120, 120a, 120b) in a first direction of rotation to subject said tensioning cable (45, 45a, 45b) to a tractive force such that it causes said movable body to move from a position distant from said fixed body, to a position close to said fixed body, in order to exert said determined substantially horizontal stretching force on said flexible curtain, which is, therefore, arranged in a laterally stretched configuration.