Current evidence suggests the occurrence of two classes of vacuolar-type H⁺-translocating inorganic pyrophosphatases (V-PPases): K⁺-insensitive proteins, identified in eukaryotes, bacteria and archaea, and K⁺-stimulated V-PPases, identified to date only in eukaryotes. Here, we describe the functional characterization of a thermostable V-PPase from the anaerobic hyperthermophilic bacterium Thermotoga maritima by heterologous expression in Saccharomyces cerevisiae. The activity of this 71-kDa membrane-embedded polypeptide has a near obligate requirement for K⁺, like the plant V-PPase, and its thermostability depends on the binding of Mg²⁺. Phylogenetic analysis of protein sequences consistently assigned the T. maritima V-PPase to the K⁺-sensitive class of V-PPases so far only known for eukaryotes. The finding of a K⁺-stimulated V-PPase also in a member of a primitive eubacterial lineage strongly supports an ancient evolutionary origin of this group of pyrophosphate-energized proton pumps.