Fe oxidation is one of Earth’s major biogeochemical processes, key to weathering, soil formation, water quality, and corrosion. However, our ability to track the contributions of Fe-oxidizing microbes is limited by our relatively incomplete knowledge of microbial Fe oxidation mechanisms, particularly in neutrophilic Fe-oxidizers. The genomes of many Fe-oxidizers encode homologs to an outer-membrane cytochrome (Cyc2) that has been shown to oxidize Fe in two acidophiles. Here, we demonstrate the Fe oxidase function of a heterologously expressed Cyc2 homolog derived from a neutrophilic Fe oxidizer. Phylogenetic analyses show that Cyc2 from neutrophiles cluster together, suggesting a common function. Sequence analysis and modeling reveal the entire Cyc2 family is defined by a unique structure, a fused cytochromeporin, consistent with Fe oxidation on the outer membrane, preventing internal Fe oxide encrustation. Metatranscriptomes from Fe-oxidizing environments show exceptionally high expression of cyc2, supporting its environmental role in Fe oxidation. Together, these results provide evidence that cyc2 encodes Fe oxidases in diverse Fe-oxidizers and therefore can be used to recognize microbial Fe oxidation. The presence of cyc2 in 897 genomes suggests that microbial Fe oxidation may be a widespread metabolism.