Most organisms performing oxygenic photosynthesis contain either cytochrome c6 or plastocyanin, or both, to transfer electrons from cytochrome b6-f to photosystem I. Even though plastocyanin has superseded cytochrome c6 along evolution, plants contain a modified cytochrome c6, the so called cytochrome c6A, whose function still remains unknown. In this article, we describe a second cytochrome c6 (the so called cytochrome c6-like protein), which is found in some cyanobacteria but is phylogenetically more related to plant cytochrome c6A than to cyanobacterial cytochrome c6. In this article, we conclude that the cytochrome c6-like protein is a putative electron donor to photosystem I, but does play a role different to that of cytochrome c6 and plastocyanin as it cannot accept electrons from cytochrome f. The existence of this third electron donor to PSI could explain why some cyanobacteria are able to grow photoautotrophically in the absence of both cytochrome c6 and plastocyanin. In any way, the Cyt c6-like protein from Nostoc sp. PCC 7119 would be potentially utilized for the biohydrogen production, using cell-free photosystem I catalytic nanoparticles.