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Metamaterials with properly engineered surface properties have been recently proposed for application in the design of broadband hybrid-mode horn antennas, such as soft and hard horns. In this paper, we present the design, fabrication, and measured results of a square dual-polarization horn antenna with thin metasurfaces lining the four walls, demonstrating broadband, negligible-loss hybrid-mode operation. By employing a powerful genetic-algorithm (GA) design optimization technique, we have dispersion-engineered low-index metaliners whose surface impedances satisfy the balanced hybrid condition across the Ku-band. The optimized metaliners were synthesized based on conventional printed-circuit board technology, leading to a lightweight and low-cost construction. To improve the cross-polarization response, a simple dielectric plug was placed in the throat of the horn to perform effective mode conversion. Measurements showed that the fabricated horn antenna prototype provided low sidelobes, low cross-polarization levels, and radiation patterns that are approximately independent of polarization. Excellent agreement was found between measured and simulated results across the entire band of operation. Both the far-field radiation patterns and the aperture field distributions confirm the hybrid-mode operation of the horn, validating the balanced metasurface design. This metamaterial-enabled antenna represents a low-cost alternative to other types of soft feed horns, such as corrugated horns.