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Copper membrane monooxygenases (CuMMOs) play critical roles in the global carbon and nitrogen cycles. Organisms harboring these enzymes perform the first, and rate limiting, step in aerobic oxidation of ammonia, methane, or other simple hydrocarbons. Within archaea, only organisms in the order Nitrososphaerales (Thaumarchaeota) encode CuMMOs, which function exclusively as ammonia monooxygenases. From grassland and hillslope soils and aquifer sediments, we identified 20 genomes from distinct archaeal species encoding divergent CuMMO sequences. These archaea are phylogenetically clustered in a previously unnamed Thermoplasmatota order, herein named the Ca. Angelarcheales. The CuMMO proteins in Ca. Angelarcheales are more similar in structure to those in ammonia-oxidizing archaea than those of bacteria, and they contain all functional residues required for activity. Similarly to the Nitrososphaerales, Ca. Angelarcheales genomes are significantly enriched in blue copper proteins (BCPs) relative to sibling lineages, including plastocyanin-like electron carriers and divergent nitrite reductase-like (nirK) 2-domain cupredoxin proteins co-located with electron transport machinery. Angelarcheales do not have identifiable genes for methanol oxidation or carbon fixation, encode significant capacity for peptide/amino acid uptake and degradation, and share numerous electron transport mechanisms with the Nitrososphaerales. In the studied soils and sediments Ca. Angelarcheales were at least as abundant as ammonia-oxidizing Nitrososphaerales. Thus, we predict that Angelarcheales live a mixotrophic lifestyle based on oxidation of ammonia liberated from peptide and amino acid degradation. This work expands the known diversity of Thermoplasmatota and of CuMMO enzymes in archaea and suggests that these organisms are important and previously unaccounted for contributors to nitrogen cycling.