The invention relates to a method for automatically matching the antenna impedance for a radiofrequency transmission circuit having an amplifier. An impedance matching network is inserted between the amplifier and the antenna. The output current i and voltage V from the amplifier and their phase shift are measured, and from this the complex impedance, defined by V/i, is deduced. The antenna impedance is calculated as a function of this complex impedance and as a function of the known present values of the adjustable impedances of the matching network. New values are calculated, from the calculated value of the antenna impedance, for the adjustable impedances of the matching network that allow an overall load impedance of the amplifier to be obtained which is as close as possible to the nominal load impedance Zopt of the amplifier, and the matching network is controlled to adjust the adjustable impedances to these new values.