Abstract: The ultrasound welding device, for welding flexible packages is disclosed. The device includes: a power generator to generate a sinusoidal electric signal at a predetermined frequency; a transducer operatively associated with the power generator to transform the electric signal generated by the latter into mechanical vibrations having the same frequency as the electric signal; a sonotrode associated with the transducer and structured to amplify and transmit the mechanical vibrations generated by the transducer to a welding surface defined on the same; and a contrast element provided with a contrast surface arranged to engage the welding surface of the sonotrode during the welding operations. The transducer and sonotrode are engaged so as to amplify and transmit the vibrations generated by the transducer to the welding surface of the sonotrode without an amplifier or similar device.