The Khone Falls and Pha Pheng Falls is a waterfall on the Mekong River in Laos in the Champasak Province near its border with Cambodia. The Khone Falls are the main reason that the Mekong is not navigable into China.

The falls' total height is 21 metres (69 ft) in segmented cascades stretching 9.7 km (6.0 mi) of the river's length. The average discharge of the cataract is nearly 11,000 m3 (388,000 cu ft) per second (3 million U.S. gallons per second), though the highest volume on record was reached at over 49,000 m3 (1,730,419 cu ft) per second (13 million U.S. gallons per second).

The area of the falls is dotted with islands and countless waterways, known as Si Phan Don (meaning 4000 islands)

The Si Phan Don ( meaning 4,000 islands) is a region of Champasak Province in southern Laos.

The Mekong river flows through the province and, just above the Cambodian border, is broken by the Khone Falls, a large number of rapids (including the Tat Somphamit rapids) which make this section impassable to river traffic. During French colonial times a narrow-gauge railway was completed in 1924 for transportation across the rapids. It was primarily used to transport gold from Laos to France. Timber was also logged in the region, and was shipped to France to build exquisite palaces. However, instead of using the railway, the timber was dragged across Done Khone with the help of elephants. In 1946 a wider track was laid and the locomotive that is currently available to tourists to see is from that time. It stands on the wider tracks and has a portion of the narrower version next to
The Mekong below the falls has a small population of Irrawaddy Dolphins. That are considered highly endangered.

In recent years the region has become a tourist destination for backpackers. The three principal islands that have been developed to cater to tourists are Don Det, Don Khon and Done Kong. Most of the lodging available on Don Det and Don Khon consists of simple, family run bungalows. Don Khong is the regional seat of administration for the area.

The Khone Falls are well known as the prime reason why the Mekong river is not navigable into China. In the late 19th century French colonialists made repeated efforts to navigate the falls but they all failed. The first time they succeeded in bringing vessels of any size to the part of the river above the falls was when they had built a railway between two islands on the falls, circumventing the steepest rapids and allowing trans-shipment.

The falls are home to the plabuck, an endangered species of catfish said to be the largest freshwater fish in the world. The plabuck is alleged to reach lengths of 3 m (10 ft) and weights of up to 646 pounds (293 kilograms).