Viruses are known as the most abundant and diverse biological entities on Earth and regarded as key ecological drivers in ecosystems. The discovery of giant viruses has challenged the conventional understanding of virology and the definition of life with their microscale-virions, megabase-genome sizes, and remarkably numerous eukaryote-specific genes, which were once considered to be hallmark genes of cellular life but barely seen in viruses. Therefore, these biological characteristics of giant viruses blur the boundary between viruses and cellular life. Metagenomics studies have revealed that giant viruses are globally distributed in marine, freshwater, and soil ecosystems, and their geographical distribution is influenced by environmental factors such as temperature, latitude, and host range. Giant virus genomes include core metabolic genes, which enhance environmental adaptability by regulating host metabolism. In addition, giant viruses may even be involved in the horizontal transfer of antibiotic resistance genes. We review the research progress in giant viruses in terms of their diversity, biogeographic distribution, ecological relationships with hosts and intracellular parasites, reprogramming of host cell metabolic systems, driving forces in biogeochemical cycles, and potential impacts on human health to explore the ecological roles of giant viruses from multiple dimensions. This review aims to revolutionize our knowledge of viruses by revealing the ecological significance of giant viruses and their roles in global biogeochemical cycles.