Nanoarchaeum equitans is a species of marine archaea that was discovered in 2002 in a hydrothermal vent off the coast of Iceland on the Kolbeinsey Ridge by Karl Stetter.

Strains of this microbe were also found on the Sub-polar Mid Oceanic Ridge, and in the Obsidian Pool in Yellowstone National Park.

Nanoarchaeum appears to be an obligate symbiont on the archaeon Ignicoccus; it must be in contact with the host organism to survive.

It lacks almost all of the genes required for the synthesis of amino acids, nucleotides, cofactors, and lipids, but encodes everything needed for repair and replication.

They argue that the initial sample, ribosomal RNA only, was biased and Nanoarchaeum actually belongs to the "Euryarchaeota" phylum.

Whether it obtains energy from biological molecules imported from Ignicoccus, or whether it receives ATP directly is currently unknown.

The genome and proteome composition of N. equitans are marked with the signatures of dual adaptation – one to high temperature and the other to obligate parasitism (or symbiosis).