An effective surface plasmon polariton (ESPP) filtering waveguide-fed D-band antenna is proposed for 6G Internet of Things (IoT) applications. Aiming at cross-band interference and link stability in 6G IoT, the design integrates an ESPP filtering structure with a waveguide slot antenna, achieving high selectivity and deep out-of-band suppression while maintaining stable radiation performance. Through ESPP dispersion engineering, the passband (135.9–143.2 GHz) and out-of-band suppression can be independently tuned. Benefiting from ESPPs’ steep asymptotic dispersion and extended modal bandgap, deep out-of-band suppression (>55 dB) and a stable stopband (147–170 GHz) are realized. Notably, the ESPP-based waveguide ensures robust performance against fabrication errors, maintaining frequency consistency between the filtering and radiation responses. The proposed ESPP-based design provides a high-selectivity and fabrication-tolerant solution, effectively mitigating complex interference in ultra-high-speed 6G IoT links and ensuring reliable operation for heterogeneous devices, such as Vehicle-to-Everything (V2X) sensors and smart city terminals.