Methylphosphonate synthase (, mpnS (gene)) is an enzyme with systematic name 2-hydroxyethylphosphonate:O<sub>2</sub> 1,2-oxidoreductase (methylphosphonate forming). This enzyme catalyses the following chemical reaction:
The carbon atom which is removed from 2-hydroxyethylphosphonic acid becomes carbonic acid. Methylphosphonate synthase is isolated from the marine archaeon Nitrosopumilus maritimus.