Cyanide is the most commonly used depressant to reject pyrite as a gangue mineral at alkaline pH in differential flotation. However, the depression mechanism of cyanide still remains uncertain. The depression effect has been proposed as the preferential adsorption of cyano species on pyrite, which has, however, not been confirmed experimentally. It is also believed that cyanide inhibits the chemisorption of thiol collectors due to the decrease of surface redox potential. In order to clarify the controversial depression mechanisms, the interactions between pyrite and cyanide at pH 10 were investigated in this study. Preferential adsorption of cyanide on pyrite rather than the decrease of surface redox potential was determined to be the depression contributor. Surface cyanoferric species were identified, both electrochemically and spectroscopically, on pyrite surface in the presence of cyanide, inhibiting the oxidation of pyrite and the oxidation/chemisorption of xanthate. This study provides new insights into the depression of pyrite flotation by cyanide.