Periodic acid (/ˌpɜːraɪˈɒdɪk/ per-eye-OD-ik) is an oxoacid of iodine. It can exist in two forms: orthoperiodic acid, with the chemical formula H₅IO₆, and metaperiodic acid, which has the formula HIO₄. Periodic acids are colourless crystals. Periodic acid features iodine in the highest oxidation state of +7.

Periodic acid was discovered by Heinrich Gustav Magnus and C. F. Ammermüller in 1833.

Modern industrial scale production involves the oxidation of a solution of sodium iodate under alkaline conditions, either electrochemically on a PbO₂ anode, or by treatment with chlorine:

A standard laboratory preparation involves treating a mixture of tribarium dihydrogen orthoperiodate with nitric acid. Upon concentrating the mixture, the barium nitrate, which is less soluble, is separated from periodic acid:

Orthoperiodic acid has a number of acid dissociation constants. The pK_a of metaperiodic acid has not been determined.

There being two forms of periodic acid, it follows that two types of periodate salts are formed. For example, sodium metaperiodate, NaIO₄, can be synthesised from HIO₄ while sodium orthoperiodate, Na₅IO₆ can be synthesised from H₅IO₆.

Orthoperiodic acid forms monoclinic crystals (space group P2₁/_n) consisting of a slightly deformed IO₆ octahedron interlinked via bridging hydrogens. Five I–O bond distances are in the range 1.87–1.91 Å and one I–O bond is 1.78 Å. The structure of metaperiodic acid also includes IO₆ octahedra, however these are connected via cis-edge-sharing with bridging oxygens to form one-dimensional infinite chains.

Orthoperiodic acid can be dehydrated to give metaperiodic acid by heating to 100 °C under reduced pressure.