The Cotter River is a fresh water river in the Australian Capital Territory. It is a tributary of the Murrumbidgee River.

It is named after a convict Garrett Cotter.

When the Government surveyor, Charles Scrivener, recommended Canberra as the best site for the national capital, water catchment was a significant consideration. Of the total of the Australian Capital Territory, were reserved as the catchment area of the Cotter River. The catchment area was calculated to support a planned population of 100,000.

There are three reservoirs on the river, the Corin, Bendora and Cotter Dams which supply water to Canberra. Most of the Cotter catchment is in the Namadgi National Park.

• Cotter Dam is a concrete gravity dam built in 1912 when the city of Canberra was established. The height of the dam wall was raised in 1951 to increase capacity. The top water level of the dam is 500.69m above sea level. The dam has a capacity of 3,856 million litres; a review in October 2006 using more accurate mapping methods resulted in capacity being re-estimated downwards from the previous estimate of 4,700 million litres. Water was pumped from the dam to Mount Stromlo and from there the water flowed by gravity to fill the city's reservoirs. The dam water quality had deteriorated compared with Bendora and Corin dams and the dam was only used when water is in short supply. However, in December 2004, ActewAGL brought the dam back on line in response to the ongoing drought.
• Bendora Dam, a double curvature concrete arch dam completed in 1961, it has a capacity of 11,540 million litres.
• Corin Dam, an earth and rockfill embankment dam completed in 1967, has a capacity of 70,900 million litres. It is the highest dam on the Cotter River. Capacity of the Bendora Dam is controlled by releasing water from the higher Corin Dam into the Cotter River.

As part of a suite of potential options for increasing water storage capacity in the ACT, a plan has been proposed to increase capacity of the Cotter Dam through the construcion of a new 76m high dam wall downstream of the existing Cotter Dam, which would inundate the existing 26m dam wall. This would increase storage capacity of the Cotter Dam from the current 4.7GL to 78GL.