The Mathias River is a river of the Canterbury Region of New Zealand's South Island. It flows from its origins in three rivers (the North, South, and West Mathias Rivers) in the Southern Alps. Of these, the North Mathias River is the longest, flowing predominantly southwards from its source northeast of Mount Williams. After eighteen kilometres its waters combine with those of the West Mathias River, which flows predominantly southwest for 13 kilometres from its sources 10 kilometres west of Mount Williams.