The River Avonbeg (Irish: Abhainn Beag, meaning "small river") is approximately 26.6 km long and flows through County Wicklow in eastern Ireland. It rises on the northern flank of Camenabologue Mountain in the Wicklow Mountains, southwest of Glendalough, and flows southeast through the Glenmalure valley, passing the villages of Greenan and Ballinaclash before joining the Avonmore River on the outskirts of the village of Avoca — a confluence known as the Meeting of the Waters.

Kayaking does occur on the Avonbeg, though it appeals primarily to experienced paddlers. The river's character is defined by twisty, rocky stretches, with Stanley Falls noted as one of the more demanding features. The narrower, calmer sections offer some relief, but the river's overgrown banks present a consistent challenge throughout, complicating access and requiring careful route planning. Access points are available near Ballinaclash.

The Glenmalure valley setting gives the river a distinctly remote, upland character. The Wicklow Way national trail runs alongside the river through this valley, and the Baravore Ford area serves as a focal point for outdoor activity in the region. Historic mining remnants are also present along the river corridor, adding some cultural and industrial heritage to the landscape.

The Avonbeg is primarily known as a spawning tributary and supports stocks of wild brown trout, though it is not a major angling destination. Angling for Atlantic salmon is prohibited on the entire Avoca River system, which includes the Avonbeg, under current regulations. Fishing for trout is controlled by riparian owners, who issue permits, with a season running from March 15 to September 30. Paddlers should be aware that riparian ownership along the banks may affect access in certain stretches.

Upland gully restoration projects were underway as of 2024, aimed at improving water quality and habitat conditions in the catchment, which may have longer-term implications for the river's recreational character.