The effect of exudates from Phaeodactylum tricornutum on the iron chemistry has been studied at pH 8.0 and 7.5 in seawater and seawater enrichment with the diatom exudates. At pH 8.0 the rate constant for the oxidation of Fe(II) decreased by 29% in the presence of the exudates, while at pH 7.5, the rate constant decreased by 56%. At the stationary phase of growth, eighteen individual phenolic compounds were identified in the extracts. The role played by the two most abundant phenols, catechin and sinapic acid, in the iron chemistry from pH 8.0 to 6.0 was investigated. These phenolic compounds favored the reduction of Fe(III) to Fe(II), which is a pH-dependent process in both catechin and sinapic acid. In the presence of catechin, the Fe(III) reduction rate, log k′ (k′, s− 1) was − 6.15 at pH 8.0 and − 3.79 at pH 6.0, in seawater. The reduction rate was lower in seawater than in NaCl solution due to the interaction of Mg2 + and, to a lesser extent, Ca2 +, with the benzoquinone intermediate. In the presence of sinapic acid, log k′ was − 6.57 at pH 8.0 and − 3.90 at pH 6.0, and was also lower in seawater than in NaCl solutions. This difference was explained by the interaction of the major ions, Ca2+ and Mg2 + with benzoquinone. The percentage of regenerated Fe(II) was always higher in the presence of catechin than in sinapic acid. This study showed that phenolic compounds exudated from diatoms can influence the iron redox chemistry and favor the persistence of Fe(II) in coastal upwelling region and in a future scenario of ocean acidification, providing possible mechanisms to make Fe available for their requirements.