Method for synchronising a gateway in a long-range wireless network affording low energy consumption, intended to enable said gateway to function according to a communication mode, known as class B, in which communication periods are defined from periodic emissions of beacons by each gateway in said network supporting said mode, referred to as class B gateways, the beacon transmissions being synchronised between each class B gateway, each communication period being divided into a predefined number of subperiods, an end device of the network functioning in accordance with said mode being able to communicate bidirectionally with a server by means of a class B gateway only during a subperiod that has been allocated to it, the method being characterised in that it comprises, when it is implemented by a first gateway : sending, to said server , by means of at least one second gateway of said network , a first frame comprising a synchronisation aid request; receiving a second frame comprising information representing a response to said request coming from a gateway in said network , referred to as the designated gateway, equipped with synchronisation means, the designated gateway having been designated by the server among the at least one second gateway, the reception of said second frame enabling the first gateway to determine at least one communication period during which the first gateway must receive a third frame, referred to as the synchronisation frame, coming from said designated gateway; obtaining information representing a subperiod of each communication period determined used by the designated gateway to transmit a synchronisation frame, said subperiod having a predefined offset from a beacon directly following the communication period; receiving at least one synchronisation frame; and for each synchronisation frame received: determining , as from an instant of receiving said synchronisation frame and the predefined offset, at least one next instant of sending a beacon for each class B gateway in said network ; and transmitting a beacon at each next transmission instant determined. sending, to said server , by means of at least one second gateway of said network , a first frame comprising a synchronisation aid request; receiving a second frame comprising information representing a response to said request coming from a gateway in said network , referred to as the designated gateway, equipped with synchronisation means, the designated gateway having been designated by the server among the at least one second gateway, the reception of said second frame enabling the first gateway to determine at least one communication period during which the first gateway must receive a third frame, referred to as the synchronisation frame, coming from said designated gateway; obtaining information representing a subperiod of each communication period determined used by the designated gateway to transmit a synchronisation frame, said subperiod having a predefined offset from a beacon directly following the communication period; receiving at least one synchronisation frame; and for each synchronisation frame received: determining , as from an instant of receiving said synchronisation frame and the predefined offset, at least one next instant of sending a beacon for each class B gateway in said network ; and transmitting a beacon at each next transmission instant determined. <EOS>