A system for fastening a telematic module to a structure of a motor vehicle, in particular to the roof of a motor vehicle, comprising: - a telematic module having a closed outer casing and containing at least one first connector for receiving signals and at least one electronic component for processing said signals; - a block for supporting said module, which is to be fixed to said structure of the motor vehicle and is equipped with a second connector designed for connection to said first connector; and - a first fastening unit and a second fastening unit, one carried by said block and the other carried by said module, or vice versa, which are configured for co-operating together to provide a connection of a removable type of said module on said block, wherein said first and second connectors (14, 22) have a configuration of a male-female type that defines a single direction (Y, Y1) of mutual connection for said first and second connectors; wherein said supporting block and said casing of said module have respective formations designed to come into mutual engagement to guide a movement of sliding of said module on said block, in said direction, as far as an end position where said first and second connectors are connected together; wherein said first unit has a seat defining at least one contrast surface, and said second unit defines a blocking element operated by a spring, said first and second units being arranged on said casing and on said supporting block in such a way that, when said module is in said end position, said blocking element is positioned in said seat pushed by said spring; and wherein said first unit or said second unit has a cam surface pre-arranged for moving said blocking element against the action of said spring as a result of the movement of said module towards said end position, said system being characterized in that: said casing or supporting block has a face in which a cavity is made, housed within which is said first connector, wherein said cavity extends on said face in said direction of connection, and wherein said block or said casing has a body that reproduces a profile corresponding to that of said cavity and is to be received therein, and said block or casing is provided with formations (24, 26), which are arranged in front of and behind said second connector or first connector with respect to said direction of connection, and have surfaces (24a, 26a) that are to come into contact with corresponding surfaces defined on the bottom of said cavity, and guides (24b, 26b) with U-shaped cross section, which are to be engaged by projecting tabs, oriented parallel to said face, which are provided on opposed side edges of said cavity. - a telematic module having a closed outer casing and containing at least one first connector for receiving signals and at least one electronic component for processing said signals; - a block for supporting said module, which is to be fixed to said structure of the motor vehicle and is equipped with a second connector designed for connection to said first connector; and - a first fastening unit and a second fastening unit, one carried by said block and the other carried by said module, or vice versa, which are configured for co-operating together to provide a connection of a removable type of said module on said block, said casing or supporting block has a face in which a cavity is made, housed within which is said first connector, wherein said cavity extends on said face in said direction of connection, and wherein said block or said casing has a body that reproduces a profile corresponding to that of said cavity and is to be received therein, and said block or casing is provided with formations (24, 26), which are arranged in front of and behind said second connector or first connector with respect to said direction of connection, and have surfaces (24a, 26a) that are to come into contact with corresponding surfaces defined on the bottom of said cavity, and guides (24b, 26b) with U-shaped cross section, which are to be engaged by projecting tabs, oriented parallel to said face, which are provided on opposed side edges of said cavity.