Perchloric acid is a mineral acid with the formula HClO₄. It is an oxoacid of chlorine. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxidizer when hot, but aqueous solutions up to approximately 70% by weight at room temperature are generally safe, only showing strong acid features and no oxidizing properties. Perchloric acid is useful for preparing perchlorate salts, especially ammonium perchlorate, an important rocket fuel component. Perchloric acid is dangerously corrosive and readily forms potentially explosive mixtures.

Perchloric acid was first synthesized (together with potassium perchlorate) by Austrian chemist Friedrich von Stadion [de] and called "oxygenated chloric acid" in the mid-1810s. French pharmacist Georges-Simon Serullas introduced the modern designation along with discovering its solid monohydrate, which he mistook for an anhydride. Berzelius produced dilute perchloric acid by electrolysis of chloric acid. In the late 1800s, German and Swedish workers commercialized the electrolysis.

Aqueous perchloric acid is produced industrially by two routes. The traditional method exploits the high aqueous solubility of sodium perchlorate (209 g/100 ml of water at room temperature). Treatment of such solutions with hydrochloric acid gives perchloric acid, precipitating solid sodium chloride:

The concentrated acid can be purified by distillation. The alternative route, which is more direct and avoids salts, entails anodic oxidation of aqueous chlorine at a platinum electrode.

In the laboratory for small-scale syntheses, it can be distilled from a solution of potassium perchlorate in sulfuric acid. It can also be synthesized by the treatment of barium perchlorate with sulfuric acid precipitating barium sulfate, leaving perchloric acid. It can also be made by mixing nitric acid with ammonium perchlorate and boiling while adding hydrochloric acid. The reaction gives nitrous oxide and perchloric acid due to a concurrent reaction involving the ammonium ion, and can be concentrated and purified significantly by boiling off the remaining nitric and hydrochloric acids.^{[citation needed]}

Anhydrous perchloric acid is synthesized by the vacuum distillation of a mixture of azeotropic aqueous perchloric acid and oleum (fuming sulfuric acid).

Anhydrous perchloric acid is an unstable oily liquid at room temperature. It forms at least five hydrates, several of which have been characterized crystallographically. These solids consist of the perchlorate anion linked via hydrogen bonds to H₂O and H₃O⁺ centers. An example is hydronium perchlorate. Perchloric acid forms an azeotrope with water, consisting of about 72.5% perchloric acid. This form of the acid is stable indefinitely and is commercially available. Such solutions are hygroscopic. Thus, if left open to the air, concentrated perchloric acid dilutes itself by absorbing water from the air.

Dehydration of perchloric acid gives the anhydride dichlorine heptoxide: