Blackwater River is the name of three rivers in the northwestern South Island of New Zealand, one flowing into the Mangles River, one into the Buller River between Westport and Inangahua Junction (named due to the dark coloured waters the Māori discovered), and one into the Māwheraiti (Little Grey) River. All three rivers are within a few kilometres of each other, to the south and east of Westport.