Marine cyanobacteria are infected by phages whose genomes encode ferredoxin (Fd) electron carriers. These Fds are thought to redirect the energy harvested from light to phage-encoded oxidoreductases that enhance viral fitness, but it is  unclear how the biophysical properties and partner specificities of phage Fds relate to those in photosynthetic organisms. Here, results of a bioinformatics analysis using a sequence similarity network revealed that phage Fds are most closely related to cyanobacterial Fds that transfer electrons from photosystems to oxidoreductases involved in nutrient assimilation. Structural analysis of myovirus P-SSM2 Fd (pssm2-Fd), which infects the cyanobacterium Prochlorococcusmarinus, revealed a high  similarity to cyanobacterial Fds (≤ 0.5 Å root-mean-square deviation). Additionally, pssm2-Fd exhibited a low midpoint reduction potential (-336 mV versus standard hydrogen electrode) similar to other photosynthetic Fds, albeit had lower thermostability (Tm = 28°C) than many other Fds. When expressed in an Escherichia coli strain deficient in sulfite assimilation, pssm2-Fd complemented bacterial growth when co-expressed with a P. marinus sulfite reductase, revealing that pssm2-Fd can transfer electrons to a host protein involved in nutrient assimilation. The high structural similarity with cyanobacterial Fds and reactivity with a host sulfite reductase suggest that phage Fds evolved to transfer electrons to cyanobacterial-encoded oxidoreductases. Published under license by The American Society for Biochemistry and Molecular Biology, Inc.