The separation of pyrite (FeS₂) from chalcopyrite (CuFeS₂) by froth flotation remains challenging in high-sulfur ore systems. Although potassium permanganate (KMnO₄) strongly depresses pyrite flotation, further improvement of its depression capacity is necessary. This study proposes a novel technique combining 120 kHz ultrasonication and KMnO₄ to inhibit FeS₂ flotation. Flotation experiments evaluated the ultrasonic effects on FeS2 depression, whereas surface characterization clarified the underlying mechanisms for the FeS₂-CuFeS₂ system through contact angle measurement, zeta potential analysis, X-ray photoelectron spectroscopy, and scanning electron microscopy. Flotation tests demonstrated that 1 min of combined conditioning effectively reduced pyrite recovery to 27.81%, significantly lower than that with KMnO4 alone (4 min). CuFeS₂ flotation by xanthate was unaffected after ultrasonication. Zeta potential analysis revealed that ultrasonication markedly increased FeS₂ surface oxidation, thus reducing xanthate adsorption. Furthermore, FeO, FeOOH, and SO₄^2‒ formed on the FeS₂ surface after combined treatment, explaining the effective depression. These findings broaden ultrasound applications in mineral separation.