A tributary (or affluent or confluent) is a contributory stream, a river that does not reach the sea, but joins another major river (a parent river), to which it contributes its waters, swelling its discharge. A tributary joins another river at a confluence. When a river's tributaries are listed in orographic sequence, they are in order from the highest (nearest the source of the river) to the lowest (nearest the mouth).

The opposite of a tributary is a distributary; a river branch that flows away from the main stream.

A river and all its tributaries drain the watershed of the river.

Network analysis examines the arrangement of tributaries in a hierarchy of 1st, 2nd, 3rd etc. orders.

In international relations history, a tributary is a region or people who pay tribute to a more powerful, suzerain state.