The efficient and safe disposition of arsenic (As)-containing copper slag depends on the accurate understanding of the As species and their corresponding leaching toxicity. The As species in copper smelting slag were investigated by X-ray fluorescence spectrometry, X-ray diffractometry, electron probe microanalyzer, scanning electron microscopy equipped with energy dispersive spectroscopy, and selectively sequential extraction scheme. Based on the accurate fractionation of As species, the potential source of As toxicity of copper slag was investigated. The results show that As can be associated with smelting slag in the forms of water-soluble As, Cu-As intermetallics, Cu-As sulfides, as well as the As incorporated in fayalite and glassy silicates. The removal of readily soluble As and Cu-As intermetallics from smelting slag by flotation can reduce the As leaching toxicity of slag tailings, making them conform to the USEPA and SEPA regulatory thresholds for As.