The angstrom or ångström is a unit of length equal to 10⁻¹⁰ m (one ten-billionth of a meter) or 0.1 nanometer. Its symbol is Å, a letter in the Swedish alphabet. The natural sciences and technology often use angstrom to express sizes of atoms, molecules, microscopic biological structures, and lengths of chemical bonds, arrangement of atoms in crystals, wavelengths of electromagnetic radiation, and dimensions of integrated circuit parts. Atoms of phosphorus, sulfur, and chlorine are about an angstrom in covalent radius, while a hydrogen atom is about half an angstrom; see atomic radius. Visible light has wavelengths in the range of 4000–7000 Å.

The unit is named after the Swedish physicist Anders Jonas Ångström (1814–1874). The symbol is always written with a ring diacritic, as the letter in the Swedish alphabet. The unit's name is often written in English without the diacritics, but the official definitions do contain them. It is not a part of the SI system of units.