Methylornithine synthase (, PylB) is an enzyme with systematic name L-lysine carboxy-aminomethylmutase. This enzyme catalyses the conversion of <small>L</small>-lysine into (3R)-3-methyl-<small>D</small>-ornithine.
The enzyme is a member of the superfamily of S-adenosyl-<small>L</small>-methionine-dependent radical enzymes.