In this study, an ultra-broadband and highly efficient polarization converter based on anisotropic resonant elements is proposed and numerically analyzed. The unit cell consists of symmetrically arranged L-shaped metallic strips integrated with a central metallic circular ring, printed on a dielectric substrate and backed by a metallic ground plane with an air spacer in between. The use of an ultra-thin dielectric substrate with a thickness of 0.6 mm, combined with a 5 mm air spacer, yields a total structure thickness of approximately 5.64 mm. This multilayer arrangement enhances resonance diversity and electromagnetic coupling, thereby improving polarization conversion performance. Simulation results show that the proposed converter achieves cross-polarization conversion efficiency above 90% across the frequency range of 4.32–13.84 GHz, corresponding to a fractional bandwidth of 105%. This range covers the entire C-, X-, and Ku-bands, demonstrating the design’s strong potential for multi-band applications. Analysis of the co-polarized and cross-polarized reflection coefficients reveals four distinct resonant modes arising from electric, magnetic, and hybrid resonance interactions. Together, these resonances significantly widen the operating bandwidth. In addition, the converter maintains stable performance under oblique incidence, confirming its wide-angle operation capability. Due to its compact, low-profile configuration and exceptional wideband performance, this converter is well-suited for integration into multi-band radar systems, high-gain satellite communication antennas, and advanced stealth technologies requiring broadband polarization manipulation.