Stream capture, river capture, river piracy or stream piracy is a geomorphological phenomenon occurring when a stream or river drainage system or watershed is diverted from its own bed, and flows down to the bed of a neighbouring stream. This can happen for several reasons, including:

The cause is not always clear.

The additional water flowing down the capturing stream may accelerate erosion and encourage the development of a canyon (gorge).

The now-dry valley of the original stream is known as a wind gap.

The Slims River was previously fed by meltwater from the Kaskawulsh Glacier in the Saint Elias Mountains in the Yukon and its waters flowed into Kluane Lake and on to the Bering Sea. Because of climate change, the glacier has rapidly receded and the meltwater no longer feeds the Slims. The water instead now feeds the Kaskawulsh River which is a tributary to the Alsek River and drains into the Gulf of Alaska.

River capture is a shaping force in the biogeography or distribution of many freshwater fish species.

Geological uplift in the southern South Island led to the divergence of freshwater galaxiid populations isolated by river capture.

The formerly massive Great Dividing Range runs the length of the eastern coastline of Australia and has isolated native freshwater fish populations east and west of the range for millions of years. In the last two million years erosion has reduced the Great Dividing Range to a critical point where west-to-east river capture events have been possible. A number of native fish species that originated in the Murray–Darling river system to the west are (or were) found naturally occurring in a number of coastal systems spanning almost the entire length of the range.