Photosystem I (PSI) is a large protein supercomplex that catalyzes the light-dependent oxidation of plas-tocyanin (or cytochrome c6) and the reduction of ferredoxin. This catalytic reaction is realized by a transmembrane electron transfer chain consisting of primary electron donor (a special chlorophyll (Chl) pair) and electron acceptors A0, A1, and three Fe4S4 clusters, FX, FA, and FB. Here we report the PSI structure from a Chl d-dominated cyanobacterium Acaryochloris marina at 3.3 ? resolution obtained by single-particle cryo-electron microscopy. The A. marina PSI exists as a trimer with three identical monomers. Surprisingly, the structure reveals a unique composition of electron transfer chain in which the primary electron acceptor A0 is com-posed of two pheophytin a rather than Chl a found in any other well-known PSI structures. A novel subunit Psa27 is observed in the A. marina PSI structure. In addition, 77 Chls, 13α-carotenes, two phylloquinones, three Fe-S clusters, two phospha-tidyl glycerols, and one monogalactosyl-diglyceride were identified in each PSI monomer. Our results provide a structural basis for deciphering the mechanism of photosynthesis in a PSI complex with Chl d as the dominating pigments and ab-sorbing far-red light.