Adenosylcobinamide-GDP ribazoletransferase (, CobS, cobalamin synthase, cobalamin-5'-phosphate synthase, cobalamin (5'-phosphate) synthase) is an enzyme with systematic name adenosylcobinamide-GDP:alpha-ribazole ribazoletransferase. This enzyme catalyses the following chemical reaction
This enzyme is part of the biosynthetic pathway to cobalamin (vitamin B<sub>12</sub>) in bacteria.