A gas burner for cooking appliances includes a cup-shaped lower support structure to be applied in a position corresponding with an opening made in the sheet metal of a cooking hob, an injector of substantially horizontal axis, applied to a lateral wall of a first cavity provided in the support structure, a substantially horizontal venturi conduit coaxial with the injector and opening at its downstream end into a second cavity provided in the support structure, open upwards, and communicating fluidly with a distribution chamber for the combustion mixture to generate a flame ring, wherein the venturi conduit includes a tubular element separate from the support structure, housed to undergo free axial movement within a corresponding seat provided in the support structure and lockable in the desired axial position by a locking member accessible from above the support structure through the opening provided in the sheet metal of the cooking hob.