The Onyx River is an Antarctic meltwater stream which flows westward through the Wright Valley from Wright Lower Glacier and Lake Brownworth at the foot of the glacier to Lake Vanda, during the few months of the Antarctic summer. Despite being only 32 kilometres (20 mi) in length it is the longest river in Antarctica.
The river flow is away from the ocean, an example of endorheic drainage, as the Wright Glacier blocks the entrance to the valley. It has several tributaries, and there are multiple meteorological stations along the length of the river. Flow levels are highly variable, both during the day and between summers, with the river failing to reach the lake some years. In contrast, it can cause significant erosion in flood years, and was rafted in 1984 by New Zealand researchers. While there are no fish in Onyx River, it does support microscopic life, and the algal blooms can be quite extensive.
The Onyx River is one of the many sites studied by the United States Antarctic Program of the National Science Foundation. The Antarctica New Zealandprogram once maintained a semi-permanent camp at Lake Vanda which has since been removed. Currently there is a small research shelter at Lake Vanda at its eastern end. Nearby is the Comprehensive Test Ban Treaty Seismic station at Bull Pass.