The device for welding thermoplastic parts includes a matrix for positioning the parts to be assembled, the matrix including an amagnetic insulating insert that defines joining zones in which the welds must be produced, a bladder defining a sealed volume and means for producing a partial vacuum in the sealed volume, and means for moving a magnetic induction head to near the joining zones and without making contact with the bladder. The welding process includes positioning at least one first part, then of placing metal inserts on areas of the first part corresponding to the joining zones that must be welded, and then positioning at least one second part. A bladder is put in place covering the parts and a partial vacuum is created in the volume defined by the bladder. The magnetic induction head is moved to produce the weld bead without contact while the partial vacuum is maintained.