Oudomxay (alternates: Oudômxai or Moung Xai; Lao: ອຸດົມໄຊ, pronounced [ʔú.dòm.sáj]) is a province of Laos, located in the northwest of the country. Its capital is Muang Xai. It covers an area of 15,370 square kilometres (5,930 sq mi). It borders China to the north, Phongsali province to the northeast, Luang Prabang province to the east and southeast, Xaignabouli province to the south and southwest, Bokeo province to the west, and Luang Namtha province to the northwest. Its topography is mountainous, between 300–1,800 metres (980–5,910 ft) above sea level.

Besides rice, other local crops are corn, soybeans, fruits, vegetables, cassava (maniok), sugarcane, tobacco, cotton wool, tea and peanuts. In 2004, approximately 10,000 tons of sugarcane and 45,000 tons of corn were produced there.

Oudomxay province, one of the provinces of Laos, covers an area of 15,370 square kilometres (5,930 sq mi). The province borders China to the north, Phongsali province to the northeast, Luang Prabang province to the east and southeast, Xaignabouli province to the south and southwest, Bokeo province to the west, and Luang Namtha province to the northwest. In the northwest there is a 15 kilometres (9.3 mi) border with the autonomous area of Xishuangbanna of the People's Republic of China. Settlements include Muang Xay, Muang La, Pak Beng, Sen Say, Taxoum, Seneke, Sala Mok, Ban Na He, Ban Chomka, Ban Lao Phe, Ban Na Houang, Ban Lo Sa, Ban Lao Than, Ban Donkon, Ban Senlouang, Ban Napa, Ban Phoukeu, Ban Yamai, Ban Haiteu, Ban Kavang, Ban Kheun, Muang Houn, Ban Na Mao, Ban Tong, Ban Khmou, Ban Khokka, and Ban Tong.

The topography of Oudomxay has altitudes vary between 300–1,800 metres (980–5,910 ft) above sea level. Approximately 60 rivers flow through Oudomxay province, as for example Nam Phak, Nam Sae, Nam Beng, Nam Kor and Nam Nga. The Nam Kor flows through the province capital Muang Xay. Oudomxay province has a moderate monsoon climate. The yearly amount of rain is about 1,900–2,600 millimetres (75–102 in). Temperatures in February and March average between 18 and 19 °C, from April to May temperatures climb over 31 °C. Due to higher altitudes there are more variations in temperature during the year and a colder dry season in northern Laos as in the rest of the country.

The Upper Lao Mekong Important Bird Area (IBA) is 10,980 hectare in size. It spans the provinces of Oudomxai, Bokeo, and Sainyabuli. The altitude is 300–400 metres (980–1,310 ft) above sea level. Noted topography includes river channel, exposed beds, sandbars, sand and gravel bars, islands, rock outcrops, bushland, and braided streams. Black-bellied Tern Sterna acuticauda, Great Cormorant Phalacrocorax carbo, Grey-headed Lapwing V. cinereus, Jerdon's Bushchat Saxicola jerdoni, Plain Martin Riparia paludicola, River Lapwing Vanellus duvaucelii, Small Pratincole Glareola lactea, and Swan Goose Anser cygnoides are some of the recorded avifauna.

Some kinds of bamboo and a range of plants (for example orchids) are found in the region. Hardwoods like teak and mahogany trees grow in Oudomxay and are sources of income for the population.

The province is made up of the following districts:

According to local history books, the first people who settled in Oudomxay around the year 700 were "Khom" (also known as Khmu).