Abstract <p>TM1347, a protein from the hyperthermophilic bacteria <i>Thermotoga maritima</i>, was assigned a putative function of an inosine 5'-monophosphate dehydrogenase (IMPDH), based on the structure and amino acid sequence alignments. IMPDH enzymes catalyze the first committed step in de novo guanine nucleotide biosynthesis, the NAD⁺-dependent oxidation of inosine 5'-monophosphate (IMP) to xanthosine 5'-monophosphate (XMP). As such, IMPDHs are the key regulators of the guanine nucleotide pool, which makes them critical for cell replication and proliferation, along with various other cellular functions such as signal transduction and energy transfer. Using a direct assay measuring the reduction of NAD⁺ to NADH, the oxidoreductase activity of TM1347 as an IMPDH was assessed under varying conditions. TM1347 was confirmed to exhibit IMPDH activity. Optimal activity of TM1347 was found to be at 80 °C, having <i>K</i>_M of 81.306 ± 16.853 μM, <i>V</i>_max of 61.554 ± 8.762 µM/s, and <i>k</i>_cat of 4.104 ± 0.584 s^–1. We report several functional similarities and differences in the function of TM1347 when comparing the kinetic parameters to analogous IMPDHs from other organisms.</p>