The crystal structure of oxygen-evolving photosystem II (PSII) has been determined at 1.9 Å resolution, providing detailed insights into its molecular architecture. PSII is a membrane protein complex in oxygenic photosynthetic organisms that catalyzes the splitting of water into protons and molecular oxygen. The structure contains 20 subunits with a total molecular mass of 350 kDa, including a Mn4CaO5 cluster that plays a central role in water oxidation. The structure reveals the precise arrangement of the Mn4CaO5 cluster, its ligands, and the positions of water molecules, which may serve as substrates for oxygen formation. Over 1300 water molecules are found in a PSII monomer, forming extensive hydrogen-bonding networks that may facilitate proton, water, and oxygen transport. The Mn4CaO5 cluster consists of four Mn atoms, one Ca atom, and five oxygen atoms, arranged in a distorted cubane-like structure. The cluster is connected to the surrounding proteins through various ligands, including amino acid side chains and cofactors. The structure also identifies the positions of chloride ions, which may help maintain the coordination environment of the Mn4CaO5 cluster. The detailed structure of PSII provides a foundation for understanding the mechanisms of water splitting and oxygen evolution, as well as the roles of various cofactors and amino acid residues in electron transfer and proton transport. The structure of PSII was determined using X-ray crystallography, with high-resolution data collected from crystals of the thermophilic cyanobacterium Thermosynechococcus vulcanus. The structure was refined to a resolution of 1.9 Å, revealing the precise arrangement of the Mn4CaO5 cluster and its ligands. The study also identifies the positions of key residues involved in electron transfer and proton transport, such as YZ (D1-Tyr161) and YD (D2-Tyr160), which are critical for the function of PSII. The structure highlights the importance of hydrogen-bonding networks in facilitating proton transport and the role of chloride ions in maintaining the structural integrity of the Mn4CaO5 cluster. The high-resolution structure of PSII provides a detailed understanding of the molecular mechanisms underlying water oxidation and oxygen evolution, which are essential for photosynthesis. This study represents a significant advance in the structural understanding of PSII and will aid in the development of new strategies for improving photosynthetic efficiency.The crystal structure of oxygen-evolving photosystem II (PSII) has been determined at 1.9 Å resolution, providing detailed insights into its molecular architecture. PSII is a membrane protein complex in oxygenic photosynthetic organisms that catalyzes the splitting of water into protons and molecular oxygen. The structure contains 20 subunits with a total molecular mass of 350 kDa, including a Mn4CaO5 cluster that plays a central role in water oxidation. The structure reveals the precise arrangement of the Mn4CaO5 cluster, its ligands, and the positions of water molecules, which may serve as substrates for oxygen formation. Over 1300 water molecules are found in a PSII monomer, forming extensive hydrogen-bonding networks that may facilitate proton, water, and oxygen transport. The Mn4CaO5 cluster consists of four Mn atoms, one Ca atom, and five oxygen atoms, arranged in a distorted cubane-like structure. The cluster is connected to the surrounding proteins through various ligands, including amino acid side chains and cofactors. The structure also identifies the positions of chloride ions, which may help maintain the coordination environment of the Mn4CaO5 cluster. The detailed structure of PSII provides a foundation for understanding the mechanisms of water splitting and oxygen evolution, as well as the roles of various cofactors and amino acid residues in electron transfer and proton transport. The structure of PSII was determined using X-ray crystallography, with high-resolution data collected from crystals of the thermophilic cyanobacterium Thermosynechococcus vulcanus. The structure was refined to a resolution of 1.9 Å, revealing the precise arrangement of the Mn4CaO5 cluster and its ligands. The study also identifies the positions of key residues involved in electron transfer and proton transport, such as YZ (D1-Tyr161) and YD (D2-Tyr160), which are critical for the function of PSII. The structure highlights the importance of hydrogen-bonding networks in facilitating proton transport and the role of chloride ions in maintaining the structural integrity of the Mn4CaO5 cluster. The high-resolution structure of PSII provides a detailed understanding of the molecular mechanisms underlying water oxidation and oxygen evolution, which are essential for photosynthesis. This study represents a significant advance in the structural understanding of PSII and will aid in the development of new strategies for improving photosynthetic efficiency.