Electric spot welding head for a multi-axis industrial robot, comprising: - a supporting structure with an end portion for attachment to the robot wrist, - a pair of welding electrodes (101, 102) carried by respective electrode-holding arms (103, 104) mounted on said supporting structure, - wherein at least one of said electrode-holding arms is movably mounted on the supporting structure of the head between an open position and a closed position, - an actuator for driving said movable arm mounted on said supporting structure and - an electrical transformer for the application of the electrical welding voltage to the welding electrodes (101, 102), having a housing with a rear wall facing towards said rear end part of attachment to the robot wrist, a front wall opposite to the rear wall, two side walls parallel to a general plane of the electrode-holding arms (103, 104) and two end walls (123, 124), said transformer further comprising an electrical connector for the connection of the power cable of the welding head, and two output poles (118, 119) of the transformer, electrically connected to the two electrode-holding arms (103, 104), wherein the two output poles (118, 119) of the transformer which are electrically connected to the two electrode-holding arms (103, 104), are arranged on different walls of the transformer casing, namely one on the front wall and the other on an end wall of the housing of said transformer, said welding head being characterized in that the structure of the welding head is completely covered by a casing consisting of two lateral half-shells (105a, 105b) coupled together, having main walls parallel to the general plane of the two electrode-holding arms, said casing having a rear opening for the connection of the supporting structure of the welding head to a flange carried by the robot wrist and a front opening from which the two electrode-holding arms (103, 104) of the welding head protrude, and in that said output poles (118, 119) of the electrical transformer have cylindrical bodies with respective internal passages (127a,127b) both supplying cooling fluid from a cooling circuit inside the transformer towards the welding electrodes (101,102), flexible fluid pipes being provided for returning the fluid which has cooled the welding electrodes to the robot without passing through the transformer. - a supporting structure with an end portion for attachment to the robot wrist, - a pair of welding electrodes (101, 102) carried by respective electrode-holding arms (103, 104) mounted on said supporting structure, - wherein at least one of said electrode-holding arms is movably mounted on the supporting structure of the head between an open position and a closed position, - an actuator for driving said movable arm mounted on said supporting structure and - an electrical transformer for the application of the electrical welding voltage to the welding electrodes (101, 102), having a housing with a rear wall facing towards said rear end part of attachment to the robot wrist, a front wall opposite to the rear wall, two side walls parallel to a general plane of the electrode-holding arms (103, 104) and two end walls (123, 124), said transformer further comprising an electrical connector for the connection of the power cable of the welding head, and two output poles (118, 119) of the transformer, electrically connected to the two electrode-holding arms (103, 104),