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Aqua fortis, or "strong water," in alchemy, is a corrosive solution of nitric acid made from saltpeter. It was used in alchemy as a solvent for dissolving silver and most other metals with notable exceptions of gold and platinum that can be dissolved using aqua regia. Aqua fortis was prepared by mixing either sand, alum, or vitriol, or the last two together, with saltpeter, then distilling it by a hot fire. The gas collected from this condenses into aqua fortis.

The discovery of aqua fortis is commonly credited to an anonymous alchemist that worked under the pen names Jabir ibn Hayyan or Geber sometime around 800 AD.

Aqua fortis was useful to refiners for parting or separating silver from gold and copper; to the workers in mosaic for staining and coloring their woods; to other artists for coloring of bone and ivory, which is done by tinging the items with copper or verdigris, then soaking in aqua fortis. Some also turn it into aqua regia, by dissolving in a fourth of its weight of sal ammoniac, and then use this to stain ivory and bone, of a fine purple color. Book binders also put it on leather, making fine marble covers for books. Diamond cutters used it to separate diamonds from metalline powders. It was also used in etching copper or brass plates. It was mixed with oil of vitriol and used to stain canes to appear like a tortoise shell by applying several coats while the cane is over hot coals. The canes were then given a gloss with a little soft wax and a dry cloth.

Aqua fortis is actually a solution of HNO₃, nitric acid, in water.

• This article incorporates content from the 1728 Cyclopaedia, a publication in the public domain.

Aqua Fortis is also mentioned in the Maquis De Sade's work, "The 120 Days Of Soddom," where it is employed as a douche.