Copper (II) sulfate (CuSO₄) is the most common copper salt, made by the action of sulfuric acid on the base copper oxide. Copper (II) sulfate may appear as a pale green or gray-white powder.

Copper (II) sulfate is a desiccant. As such, it most commonly occurs as the blue hydrate mineral chalcanthite, CuSO₄ · 5H₂O, which dehydrates at 110°C. It decomposes at 600°C forming CuO and Cu₂O.

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).

The archaic name for Copper (II) sulfate was Cupric sulfate. Some common names are: Blue Stone and Blue Vitriol.