Arsenic acid is a chemical compound. It is also an acid. Its chemical formula is H₃AsO₄. It has hydrogen and arsenate ions in it.

Arsenic acid is a colorless solid. It absorbs water from the air. It is a strong oxidizing agent. It is similar to phosphoric acid. It can be deprotonated three times, making various arsenates. Arsenic acid reacts with iodide to make iodine.

It is made by reacting moist arsenic with ozone or by reacting arsenic trioxide with concentrated nitric acid. It can also be made by dissolving arsenic pentoxide in water.

Arsenic acid is used in the making of some dyes and organic arsenic compounds. It is also used as a preservative for wood and a biocide (something that kills all living things).

• Arsenous acid
• Arsenic trichloride
• Arsine