Arsenous acid, also known as arsenious acid, is a chemical compound. It is also an acid. Its chemical formula is As(OH)₃. It has arsenic and hydroxide ions in it. The arsenic is in its +3 oxidation state.

Arsenous acid is a colorless solution. It is a weak acid. It reacts with bases to make arsenite salts. It is highly toxic. It cannot be made in a crystalline form because it turns into arsenic trioxide and water again. It is similar to phosphorous acid, although it is not a strong reducing agent.

It is made by dissolving arsenic trioxide or arsenic trichloride in water.

• Arsenic acid
• Arsenic tribromide
• Arsenic trisulfide
• Nitrous acid