Methionine-S-oxide reductase (, methyl sulfoxide reductase I and II, acetylmethionine sulfoxide reductase, methionine sulfoxide reductase, L-methionine:oxidized-thioredoxin S-oxidoreductase) is an enzyme with systematic name L-methionine:thioredoxin-disulfide S-oxidoreductase. This enzyme catalyses the following chemical reaction
In the reverse reaction, dithiothreitol can replace reduced thioredoxin.
S-Methionine-S-oxide and R-Methionine-S-oxide require different Methionine-S-oxide reductases, named Methionine-S-oxide reductase A and Methionine-S-oxide reductase B respectively. A Methionine-S-oxide reductase B named AtMSRB plays a large role in salt tolerance in Arabidopsis Thaliana.