Catalyzes the methylation of DL- and L-selenocysteine with S-methylmethionine as donor. Methylates also DL-homocysteine, DL- and L-cysteine in vitro. May be involved in selenium detoxification. L-selenocysteine + S-methyl-L-methionine = H(+) + L-methionine + Se-methyl-L-selenocysteine Inhibited by L-methionine. Optimum pH is 7.0. Expressed in roots, young leaves and florets, but not detected in plants not exposed to selenium. Up-regulated by selenate, but not by selenite. Down-regulated by sulfate.