A method of authenticating the user (U) of a terminal (2) connected to an Internet type network. A communications channel is set up in a secure mobile telephony network (GSM) between mobile equipment (3) of the user (U) situated close to the terminal (2) and an authentication unit (1). The terminal (2) downloads (E30) an auto-run digital code from the authentication unit (1). The mobile equipment sends (E40) a sound signal via the mobile communications channel to the authentication unit (1), the sound signal being generated (E35) by the terminal (2) on the basis of the digital code. The user (U) is authenticated on the basis of the sound signal received (E40) via the mobile communications channel (GSM), and on the basis of an identifier (GSM_No) of the mobile equipment (3).