Atomic and molecular iodine, I˙ and I2, play important roles in the atmosphere, such as the catalytic depletion of ozone and the oxidation of gaseous elemental mercury. It is known that the major source of I˙ and I2 in the atmosphere is the photodissociation of organoiodine molecules released by algae in the sea. In this study, we show the striking results of the spontaneous and ultrafast oxidation of I- into I˙, which further evolves into I2- and I3- in water microdroplets, presenting a previously unknown source of I˙ and I2 in atmospheric water, such as the sea spray or cloud microdroplets. Mass spectrometric evidence shows that spontaneously generated hydroxyl radicals in water microdroplets are responsible for the oxidation of I-. Taken together, we opine that microdroplet chemistry may adopt significant roles in atmospheric redox chemistry.