Specifically hydrolyzes (R)-S-adenosyl-L-methionine ((R)-SAM), the inactive form of the ubiquitous cofactor SAM, into adenosine and L-methionine. Is stereoselective as it cannot use the active form of SAM, (S)-S-adenosyl-L-methionine, as substrate. Likely plays a role in preventing accumulation of (R)-S-adenosyl-L-methionine in cells; maintenance of (S)-S-denosyl-L-methionine homochirality is important for cellular health given that the (R)-form is largely inactive as a methyl donor and can function as an inhibitor of methyltransferases. (R)-S-adenosyl-L-methionine + H2O = adenosine + H(+) + L-methionine kcat is 0.0045 sec(-1). Belongs to the SAM hydrolase / SAM-dependent halogenase family.