In enzymology, a L-methionine (R)-S-oxide reductase () is an enzyme that catalyzes the chemical reaction
The 3 substrates of this enzyme are L-methionine, thioredoxin disulfide, and H<sub>2</sub>O, whereas its two products are L-methionine (R)-S-oxide and thioredoxin.
This enzyme belongs to the family of oxidoreductases, specifically those acting on a sulfur group of donors with a disulfide as acceptor. The systematic name of this enzyme class is L-methionine:thioredoxin-disulfide S-oxidoreductase [L-methionine (R)-S-oxide-forming]. Other names in common use include fRMsr, FRMsr, free met-R-(o) reductase, and free-methionine (R)-S-oxide reductase. This enzyme participates in methionine metabolism.