Fastening device for a bracelet, said fastening device being suited to be connected to a first terminal area and to a second terminal area of said bracelet, said fastening device comprising a first laminar element and a second laminar element mutually hinged to each other and suited to be arranged in a closed operating configuration, in which they are superimposed, and in an open operating configuration, in which they are at least partially parted, said first laminar element comprising a first area suitable for connection to said first terminal area of said bracelet and a hinge area where it is hinged to said second laminar element, said second laminar element comprising a hinge area where it is hinged to said first laminar element, said fastening device further comprising adjusting means suited to adjust the distance between said first terminal area and said second terminal area of said bracelet, said adjusting means comprising: - a coupling element suited to be connected to said second terminal area of said bracelet; - first connection means associated with said coupling element; - second connection means defined in said second laminar element; said first and said second connection means (64, 66) being suited to cooperate in order to arrange said coupling element in several positions with respect to said second laminar element, further comprising holding means suited to maintain said coupling element coupled with said second laminar element, and characterized in that said holding means comprise an auxiliary laminar element (82; 82'; 82"; 82′′′) hinged to said second laminar element, said auxiliary laminar element (82; 82'; 82"; 82′′′) being suited to be arranged in a closed holding position on said second laminar element and said coupling element, and in an open position suited to allow said coupling element to be disconnected from said second laminar element. a first laminar element and a second laminar element mutually hinged to each other and suited to be arranged in a closed operating configuration, in which they are superimposed, and in an open operating configuration, in which they are at least partially parted, said first laminar element comprising a first area suitable for connection to said first terminal area of said bracelet and a hinge area where it is hinged to said second laminar element, said second laminar element comprising a hinge area where it is hinged to said first laminar element, said fastening device further comprising adjusting means suited to adjust the distance between said first terminal area and said second terminal area of said bracelet, said adjusting means comprising: - a coupling element suited to be connected to said second terminal area of said bracelet; - first connection means associated with said coupling element; - second connection means defined in said second laminar element; - a coupling element suited to be connected to said second terminal area of said bracelet; - first connection means associated with said coupling element; - second connection means defined in said second laminar element; said first and said second connection means (64, 66) being suited to cooperate in order to arrange said coupling element in several positions with respect to said second laminar element, further comprising holding means suited to maintain said coupling element coupled with said second laminar element, and characterized in that said holding means comprise an auxiliary laminar element (82; 82'; 82"; 82′′′) hinged to said second laminar element, said auxiliary laminar element (82; 82'; 82"; 82′′′) being suited to be arranged in a closed holding position on said second laminar element and said coupling element, and in an open position suited to allow said coupling element to be disconnected from said second laminar element.