Ammonia-oxidizing Archaea of the phylum Nitrososphaerota are one of the most abundant microbial groups responsible for a large part of the primary production in the dark ocean. These microorganisms use the energy-efficient variant of the 3-hydroxypropionate/4-hydroxybutyrate cycle to assimilate inorganic carbon. In this variant of the cycle, only a few enzymes have been identified biochemically, while most of the specific dehydrogenases of the cycle have not yet been identified. Here, we demonstrate that gene nmar_0523 encodes succinic semialdehyde reductase in Nitrosopumilus maritimus. Characterization of the heterologously produced succinic semialdehyde reductase revealed that it is a homotrimeric enzyme specific for succinic semialdehyde as a substrate. This enzyme is not homologous to other identified succinic semialdehyde reductase, highlighting the convergent evolution of the 3-hydroxypropionate/4-hydroxybutyrate cycle in hyperthermophilic Sulfolobales (Thermoproteota) and in mesophilic ammonia-oxidizing Archaea.