A method is reported to suppress the sidelobe radiation of a standard pyramidal horn using a thin single-layer metamaterial lens. The metamaterial lens is composed of two identical metallic dipole arrays printed on both sides of a single dielectric layer. By placing the metamaterial lens inside a standard pyramidal horn, the electromagnetic fields over the horn aperture can be modified, resulting in low-sidelobe radiation in both the E- and H-planes of the horn. The measurement of a prototype lens gave a 10 dB reduction of the first sidelobe level in the E-plane, compared with that of the original air-filled pyramidal horn. Other design possibilities with a compensating lens are suggested.