Viruses dominate the deep-ocean biosphere (>200-m depth) yet remain profoundly understudied. As the most abundant biological entities on Earth, they govern the geochemistry, evolution, and ecology of deep-sea prokaryotic and eukaryotic communities across a vast array of deep-sea biomes. The last two decades have witnessed revolutionary advances in deep-sea sampling, sequencing, and computational biology, illuminating viral diversity and function across Earth's largest active biome. Here, we synthesize emerging patterns in deep-sea viral ecology, biogeography, and evolutionary dynamics while identifying critical knowledge gaps and opportunities. This review is pertinent in the face of deep-sea mining and climate change, which threaten to fundamentally alter these ecosystems and cascade through virus-host networks in ways we cannot yet predict. Understanding viral control of deep-sea biogeochemistry and resistance to environmental perturbation is essential for predicting ecosystem resilience in the face of anthropogenic pressures and informing policy on one of Earth's final frontiers.
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