Abstract: A security device for an intrusion-alarm system, which comprises: radio-frequency detection means (10); means (11) for generating a signal indicating a possible sabotage, when the radio-frequency detection means detect a radio frequency over a defined time; means (12) for detecting an intrusion-indicating condition; means (13) for generating an intrusion-indicating signal when the means for detecting an intrusion-indicating condition detect an intrusion-indicating condition; and means (14) for generating an alarm-raising signal, in the event that the signal indicating a possible sabotage and the intrusion-indicating signal are generated. The invention also relates to an intrusion-alarm system which comprises such a device.