Copper(II) sulfate (CuSO₄) is the most common copper salt, made by the action of sulfuric acid on the base copper(II) oxide. The anhydrous form is a pale green or gray-white powder, while the hydrated form is bright blue. The archaic name for copper(II) sulfate was cupric sulfate. Some common names are: blue stone and blue vitriol.

Copper(II) sulfate is a desiccant. As such, it most commonly occurs in nature as copper(II) sulfate pentahydrate (CuSO₄ · 5H₂O). This mineral is called chalcanthite. Copper(II) sulfate decomposes before melting. The common pentahydrate form dehydrates, losing four water molecules at 110 °C and all five at 150 °C. At 650 °C copper(II) sulfate decomposes into copper(II) oxide (CuO), Sulfur dioxide (SO₂) and oxygen (O₂).

It can be used to plate metals with copper, as a fungicide or herbicide, or as a chemical test for water (the anhydrous form will absorb water, turning blue).