The River Lambourn is a chalk stream in the English county of Berkshire. It rises in the Berkshire Downs near its namesake village of Lambourn and is a tributary of the River Kennet, which is itself a tributary of the River Thames.

Chalk streams are rivers that rise from springs in landscapes with chalk bedrock. Since chalk is permeable, water percolates easily through the ground to the water table and chalk streams therefore receive little surface runoff. As a result, the water in the streams contains little organic matter and sediment and is generally very clear.