A completely nitrifying bacterium, Candidatus Nitrospira inopinata, was discovered and cultivated from the genus Nitrospira, which are typically nitrite oxidizers. This organism can perform both ammonia and nitrite oxidation, completing the nitrification process. The genome of this organism encodes both pathways for ammonia and nitrite oxidation, which are expressed simultaneously during growth. The discovery indicates that Nitrospira can carry out complete nitrification, challenging previous assumptions about the separation of nitrification steps in different organisms. This finding suggests that Nitrospira are key components in nitrogen cycling microbial communities. The organism was identified through enrichment cultures and metagenomic analysis, revealing its ability to oxidize ammonia to nitrate and its genetic makeup supporting both ammonia and nitrite oxidation. The study also highlights the potential for comammox organisms, which can perform complete nitrification, to exist in nature, with implications for understanding nitrogen cycling and microbial ecology. The organism was found in various environments, including engineered systems, and its presence was confirmed through multiple analyses, including metagenomics, proteomics, and physiological experiments. The discovery has significant implications for the microbiology of nitrogen cycling and the understanding of microbial interactions in nitrification processes.A completely nitrifying bacterium, Candidatus Nitrospira inopinata, was discovered and cultivated from the genus Nitrospira, which are typically nitrite oxidizers. This organism can perform both ammonia and nitrite oxidation, completing the nitrification process. The genome of this organism encodes both pathways for ammonia and nitrite oxidation, which are expressed simultaneously during growth. The discovery indicates that Nitrospira can carry out complete nitrification, challenging previous assumptions about the separation of nitrification steps in different organisms. This finding suggests that Nitrospira are key components in nitrogen cycling microbial communities. The organism was identified through enrichment cultures and metagenomic analysis, revealing its ability to oxidize ammonia to nitrate and its genetic makeup supporting both ammonia and nitrite oxidation. The study also highlights the potential for comammox organisms, which can perform complete nitrification, to exist in nature, with implications for understanding nitrogen cycling and microbial ecology. The organism was found in various environments, including engineered systems, and its presence was confirmed through multiple analyses, including metagenomics, proteomics, and physiological experiments. The discovery has significant implications for the microbiology of nitrogen cycling and the understanding of microbial interactions in nitrification processes.