Monoheme cytochromes of the C-type are involved in a large number of electron transfer processes, which play an essential role in multiple pathways, such as respiratory chains, either aerobic or anaerobic, and the photosynthetic electron transport chains. This study reports the biochemical characterization and the crystallographic structure, at 1.23 A resolution, of a monoheme cytochrome c from the thermohalophilic bacterium Rhodothermus marinus. In addition to an alpha-helical core folded around the heme, common for this type of cytochrome, the X-ray structure reveals one unusual alpha-helix and a unique N-terminal extension, which wraps around the back of the molecule. Based on a thorough structural and amino acid sequence comparison, we propose R. marinus cytochrome c as the first characterized member of a new class of C-type cytochromes.