The Cajari River () is a river of Marajó, which itself is an island in the Amazon Delta. It is located in the state Pará in northern Brazil.
The Cajari River branches off from the Cajuúna River. It drains the center of the Rio Cajari Extractive Reserve, created in 1990. The upper and middle course of the Cajari form flooded terraces. In the lower course the river merges with other water bodies, which form meanders, lakes and channels.