Abstract: Herein a gas boiler burner (1) is described comprising a diffuser, suitable for diffusing premixed fuel gases in a combustion chamber, in which the diffuser comprises a diffuser central portion (10) and two diffuser longitudinally peripheral portions (5), the two diffuser longitudinally peripheral portions (5) being connected together through the- central diffuser portion (10), the central diffuser portion (10) extending longitudinally in a direction perpendicular to the flow of the fuel gases over a length l greater than zero and having transversal cross- sections shaped as a circumference arc having a uniform radius of curvature r, each of the diffuser longitudinally peripheral portions (5) being shaped as a portion of a sphere having radius r.