Diatomic molecules are molecules composed only of two atoms, of either the same or different chemical elements. The prefix di- is of Greek origin, meaning 2. Common diatomic molecules are hydrogen (H₂), nitrogen (N₂), oxygen (O₂), and carbon monoxide (CO). Seven elements exist as homonuclear diatomic molecules at room temperature: H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂. Many elements and chemical compounds aside from these form diatomic molecules when evaporated. The noble gases do not form diatomic molecules: this can be explained using molecular orbital theory (see molecular orbital diagram).

Hundreds of diatomic molecules have been characterized in the terrestrial environment, laboratory, and interstellar medium. About 99% of the Earth's atmosphere is composed of diatomic molecules, specifically oxygen and nitrogen at 21% and 78%, respectively. The natural abundance of hydrogen (H₂) in the Earth's atmosphere is only on the order of parts per million, but H₂ is, in fact, the most abundant diatomic molecule in nature. The interstellar medium is, indeed, dominated by hydrogen atoms.