An automatic antenna impedance matching method for a radiofrequency transmission circuit. An impedance matching network is inserted between an amplifier and an antenna. The output current and voltage of the amplifier and their phase difference are measured by a variable measurement impedance, and the complex load impedance of the amplifier is deduced from this; the impedance of the antenna is calculated as a function of this complex impedance and as a function of the known current values of the impedances of the matching network. Starting from the value found for the impedance of the antenna, new values of the matching network are calculated that allow the load to be matched to the nominal impedance of the amplifier. The measurement impedance has a value controllable by the calculation processor according to the application and notably as a function of the operating frequency and of the nominal impedance of the amplifier.