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MtxX, also known as Methanogen Marker Protein 4 (MMP4), is a member of the group of proteins conserved in archaeal methanogens called the Methanogen Marker Proteins (MMPs). Owing to this taxonomic distribution the MMPs are presumed to have roles related to methanogenesis or are evidence for an evolutionary history associated with methanogenic processes. MtxX is sequence-annotated as either a methyltransferase (EC 2.1.1.-) or a phosphate acetyl/butyryltransferase (EC 2.3.1.8/2.3.1.19). Gene synteny analysis shows mtxX is located next to other MMP genes in Methanomicrobiales, Methanotrichales, and Methanocaldococcus genomes, while in Methanobacteria and Methanococci it is positioned adjacent to undecaprenyl pyrophosphate synthase, a cell wall biosynthesis enzyme. We describe the crystal structure for MtxX from Methanothermobacter thermautotrophicus DeltaH showing that it has a protein fold homologous to phosphate acetyltransferases and decarboxylating NAD(P)-dependent dehydrogenases. The MtxX structure has a conserved binding cleft which is the presumptive functional site based on crystallographic symmetry-related molecular binding interactions and structural homology.