The study reports the discovery and cultivation of a completely nitrifying bacterium from the genus *Nitrospira*, a globally distributed group of nitrite oxidizers. The genome of this chemolithoautotrophic organism encodes both the pathways for ammonia and nitrite oxidation, which are concomitantly expressed during growth by ammonia oxidation to nitrate. The findings suggest that *Nitrospira* can perform complete nitrification, a process that has been considered energetically advantageous but not observed in nature. The study also reveals the presence of genes affiliated with the phylogenetically distinct ammonia monooxygenase and hydroxylamine dehydrogenase genes of *Nitrospira* in many environments, indicating that comammox (complete ammonia oxidizer) organisms may be more common in nature than previously thought. The discovery of comammox *Nitrospira* has significant implications for our understanding of nitrogen cycling and the role of these organisms in engineered systems.The study reports the discovery and cultivation of a completely nitrifying bacterium from the genus *Nitrospira*, a globally distributed group of nitrite oxidizers. The genome of this chemolithoautotrophic organism encodes both the pathways for ammonia and nitrite oxidation, which are concomitantly expressed during growth by ammonia oxidation to nitrate. The findings suggest that *Nitrospira* can perform complete nitrification, a process that has been considered energetically advantageous but not observed in nature. The study also reveals the presence of genes affiliated with the phylogenetically distinct ammonia monooxygenase and hydroxylamine dehydrogenase genes of *Nitrospira* in many environments, indicating that comammox (complete ammonia oxidizer) organisms may be more common in nature than previously thought. The discovery of comammox *Nitrospira* has significant implications for our understanding of nitrogen cycling and the role of these organisms in engineered systems.