Iodine is widely distributed in Nature, occurring as iodides or iodates but never as a free element. It is usual to find iodine in rocks, soils and underground brines. Seawater contains about 0.05 ppm. Less than a dozen minerals in which iodine is an essential constituent have been found being the most important lautarite and calcium iodate, found in Chilean nitrate deposits.

Despite the low concentrations of iodine in seawater, some seaweeds, notably of the brown variety, accumulate iodine on a dry basis.