A support bracket for cable trays, said support bracket being formed by a one-piece body made of a polymer material and comprising: - a base for fixing said support bracket to a vertical support surface; and - an arm extending in a cantilever manner from said base and comprising: an upper wall with an oblong upper face extending in a horizontal plane in a horizontal longitudinal direction of said arm, with respect to a usage position of said support bracket in which said support bracket is fixed by said base to said vertical support surface, said upper face of the arm forming a seating surface for a cable tray which is seated on said upper face of the arm; a lower wall opposite said upper wall; and a central partition joining said upper wall and said lower wall; wherein said central partition has at least one through opening adjacent to said upper wall; characterised in that said upper wall is provided with through holes (6a, 6b) for the passage of fixing screws for fixing the cable tray to said support bracket; and said at least one through opening is located below at least one of said through holes (6a, 6b), such that said through opening contains an empty space into which said through hole (6a, 6b) comes, said empty space being a rectangular parallelepiped with a width, in said horizontal longitudinal direction of the arm, greater than or equal to 4 cm, and a height, in a vertical direction orthogonal to said horizontal longitudinal direction, greater than or equal to 2 cm. - a base for fixing said support bracket to a vertical support surface; and - an arm extending in a cantilever manner from said base and comprising: an upper wall with an oblong upper face extending in a horizontal plane in a horizontal longitudinal direction of said arm, with respect to a usage position of said support bracket in which said support bracket is fixed by said base to said vertical support surface, said upper face of the arm forming a seating surface for a cable tray which is seated on said upper face of the arm; a lower wall opposite said upper wall; and a central partition joining said upper wall and said lower wall; wherein said central partition has at least one through opening adjacent to said upper wall; characterised in that said upper wall is provided with through holes (6a, 6b) for the passage of fixing screws for fixing the cable tray to said support bracket; and said at least one through opening is located below at least one of said through holes (6a, 6b), such that said through opening contains an empty space into which said through hole (6a, 6b) comes, said empty space being a rectangular parallelepiped with a width, in said horizontal longitudinal direction of the arm, greater than or equal to 4 cm, and a height, in a vertical direction orthogonal to said horizontal longitudinal direction, greater than or equal to 2 cm. a lower wall opposite said upper wall; and a central partition joining said upper wall and said lower wall; wherein said central partition has at least one through opening adjacent to said upper wall; characterised in that said upper wall is provided with through holes (6a, 6b) for the passage of fixing screws for fixing the cable tray to said support bracket; and said at least one through opening is located below at least one of said through holes (6a, 6b), such that said through opening contains an empty space into which said through hole (6a, 6b) comes, said empty space being a rectangular parallelepiped with a width, in said horizontal longitudinal direction of the arm, greater than or equal to 4 cm, and a height, in a vertical direction orthogonal to said horizontal longitudinal direction, greater than or equal to 2 cm.