The Xi River (/ʃiː/; Chinese: 西江) or Si-Kiang is the western tributary of the Pearl River in southern China. It is formed by the confluence of the Gui and Xun Rivers in Wuzhou, Guangxi. It originates from the eastern foot of the Maxiong Mountain in Qujing City, Yunnan Province. Then it flows east through Guangdong, and enters the Pearl River Delta just east of the Lingyang Gorge in Zhaoqing. The main branch of the Xi River flows southeast through the delta entering the South China Sea at Modao Men, just west of Macau. The major cities along the Xi include Wuzhou, Zhaoqing, and Jiangmen.

The other two main tributaries of Pearl River are the Dong River (literally, the East River) and Bei River (the Northern River). As for other functions, it plays a vital role in carbon storage and transport in Southern China. The Xi River is facing some ecological challenges such as drought, invasive species, and pollution.

The basin of the Xi River ranges from eastern Yunnan Province to southern Guangdong Province with a humid subtropical or tropical monsoon climate. The Xi River is navigable for its entire length. It is a commercial waterway of southern China, and links the delta cities to the interior. Over two thousand years ago, the Lingqu Canal was dug, connecting the Xi River basin (the Li River, which is a tributary of the Gui River) with the Xiang River, which flows into the Yangtze, thus providing a continuous waterway from the Pearl River Delta to the Yangtze Valley. The Xi River is rich in water conservancy and water resources, providing significant contributions to agricultural irrigation, river transportation, and power generation in coastal areas.

The Xi River is the largest of the Pearl's tributaries. Its volume of flow is second in China only to that of the Yangtze River, and it supplies water to many places in Guangxi, Guangdong and Macau. The greater Xi River is also one of China's longest. Existing in many segments it extends for 2,271.8 km (1,411.6 mi):

Rivers are a significant storage and transport system for both organic and inorganic carbon. The Xi River is an important source of atmospheric carbon dioxide in Southern China, with carbon inputs coming mainly from the river's headwater Nanpan and Beipan rivers. In the Beipan River, carbon inputs come from the oxidation of organic carbon and the coal industry along the river. In the Nanpan River, carbon inputs come mainly from soil and organic detritus.

The Xi River basin has historically experienced droughts, which are worsening due to climate change and rapid urbanization along with many parts of the river. This increase in population is straining water resources. The climate of the watershed is subtropical to tropical monsoon, with 80% of precipitation falling between April and September. This extreme variation in yearly precipitation patterns exacerbates drought issues. Climate change is predicted to cause a significant decrease in average, highest, and lowest river flows, with average flow droppings anywhere from 4 to 49%. Increasing frequency and severity lead the meteorological and hydrological droughts.

Invasive or non-native species are an issue in rivers in southern China, including the Xi River. Southern China rears the majority of non-native species in aquaculture. These species are mainly introduced from aquaculture, and may predate or outcompete native species. Most introduced species are omnivores, including the Nile tilapia, the most common non-native species in southern China rivers.

Many commercial fish species in the Xi River and the encompassing Pearl River watershed have been contaminated with significant amounts of Bisphenol A, common in plastics manufacturing. Bisphenol A is a chemical that can be harmful to the endocrine system of fish and humans.