This research develops a novel manufacturing approach for millimeter-wave feedhorns, utilizing additive manufacturing combined with electroless metallization. A corrugated horn antenna operating across the K-band spectrum was engineered and produced using polymer-based 3D printing, followed by internal surface silver deposition. This methodology achieved complex internal geometry consolidation in a single process, yielding a structure with merely 17% the mass of comparable steel counterparts. Electrical characterization demonstrated reflection coefficients predominantly exceeding −20 dB magnitude across 18.0–27.0 GHz alongside the attained gain values surpassing 14 dB within 18.0–24.0 GHz. The measured far-field radiation characteristics showed excellent correlation with computational electromagnetic models. The demonstrated technique presents transformative potential for the mass-efficient production of high-frequency components in next-generation small satellite constellations and compact radar platforms.