Disclosed is a resistance welding device (1), especially for welding thermoplastics, which comprises an actuator (2) of elongated shape with a distal end (3) and a proximal end (4), a heating system with a transformer (7), a cooling system and an electronic power supply and control system (6). The actuator (2) has a power supply at its distal end (3) and at its proximal end (4) it has an electrode system (81, 82) terminated by a heating element (9) at the outlet (52) of the cooling system. The heating system with a transformer (7), the cooling system and the electronic power supply and control system (6) are located inside the actuator (2). Additionally, the transformer (7) is connected to the heating element (9).