Abstract: Owing to the global consumption of high-grade ores, large amounts of low-grade and complex ores are being processed nowadays. The entrainment of gangue minerals in the ores, especially silicate gangue minerals, has become a nonignorable issue that can significantly influence mineral flotation performance. Amorphous silica has been reported to exist widely in various ore deposits, but how it affects mineral flotation remains poorly understood. This study investigated the rheological properties and entrainment behaviors of quartz/amorphous silica gangue in copper flotation through batch flotation tests, rheology measurements, cryo-SEM analysis, and particle settlement tests. Results indicate an exponential increase in the pulp viscosity as the amorphous silica content in the gangue increased. These also led to distinct recoveries of chalcopyrite and gangue minerals in flotation: the chalcopyrite recovery kept decreasing, whereas the entrainment recovery of gangue minerals increased initially and then decreased. The change in the entrained gangue recovery was found to be a consequence of the reduced water recovery and the increased degree of entrainment for the gangue. In a range wherein the viscosity growth was low, the increased degree of entrainment for the gangue prevailed over the decreased water recovery, ultimately leading to an increase in the gangue recovery. However, when the pulp viscosity drastically increased, the decreased water recovery was found to be dominant in changing the gangue recovery, and thus the gangue recovery decreased. Further, the effect of amorphous silica on the degree of entrainment for the gangue was investigated on a size-by-size basis. A great difference was found in the degree of entrainment for the gangue mineral particles of the same size fraction. However, fine gangue mineral particles experienced the greatest increase in their entrainment degree. Therefore, the degree of entrainment for the gangue was size-dependent and subject to the presence of amorphous silica. The results of cryo-SEM and settling tests indicate that quartz/amorphous silica particle aggregates were formed when amorphous silica particles were present in the flotation system, and these aggregates had a relatively low sedimentation rate. It was inferred that formed gangue aggregates increased the pulp viscosity, which reduced the water drainage in the froth and the sedimentation rate of gangue particles, subsequently increasing the mass of the gangue in a unit mass of water and, thus, the degree of entrainment.