Terahertz (THz) polarization converters are essential components for advancing THz applications in imaging, sensing, and high-speed communications. However, achieving both broad bandwidth and high conversion efficiency remains a significant challenge. In this work, we propose, fabricate, and experimentally validate a transmissive linear polarization converter (TTPC) operating in the terahertz band by utilizing a bilayer metallic metamaterial structure. The device consists of a top-layer metasurface with a square patch and split-ring resonator and a bottom-layer metallic grating, separated by a polyimide substrate. Through full-wave electromagnetic simulations and surface current analysis, we reveal that the high-performance broadband polarization conversion arises from the synergistic interaction among three distinct resonance modes. Stokes parameter analysis further confirms that the polarization rotation angle remains stable at approximately 90°, with near-linear output across the operational band. Experimental characterization using a terahertz time-domain spectroscopy (THz-TDS) system demonstrates that the device achieves a polarization conversion ratio (PCR) exceeding 92% over a broad frequency range of 0.53–1.77 THz, corresponding to a relative bandwidth of 108%. The measured insertion loss varies between 5.5 dB and 12 dB within the operating band, which is attributed to ohmic loss, dielectric absorption, and resonant energy dissipation. Despite these losses, the converter maintains high polarization purity and practical utility. With a compact and fabrication-friendly architecture, the proposed TTPC provides a viable route to high-performance, broadband polarization control in terahertz systems, thereby paving the way for its integration into next-generation THz communication and imaging devices.