A multi flame-ring gas burner for cooktops includes a lower burner body configured to receive at least two injectors facing Venturi ducts; a first burner base with a first cap and with openings for a central flame ring; and a second burner base with a second annular cap and openings for an outer annular flame ring. The lower burner body has a seat for a spark plug interacting with the first burner base; two cylindrical seats having substantially horizontal and parallel axes accommodating two tubular elements internally shaped like linear Venturi ducts; and a substantially tubular portion projecting upward, the first burner base being integral with the projecting tubular portion. An injector-holder unit is applicable to the lower burner body and has two portions that are removably insertable into corresponding through holes in the lower burner body and facing the tubular elements internally shaped like linear Venturi ducts.