We report the synthesis of a mer-triaqua Rh-III complex using terpyridine as a meridional tridentate ligand. This is the first example of a structurally characterized six-coordinated mer-triaqua complex for any second- or third-row element from the main transition-metal groups (groups 5-11). This is also the first example of a structurally characterized mer-triaqua complex with terpyridine for any transition metal. Acid dissociation constants, pK(a1) and pK(a2), of the triaqua complex are determined to be 3.0 and 6.9, respectively.