Chiang Rai (Thai: เชียงราย, pronounced [tɕʰīaŋ rāːj]; Northern Thai: ᨩ᩠ᨿᨦᩁᩣ᩠ᨿ, pronounced [tɕīaŋ hāːj]) is one of Thailand's seventy-six provinces that lies in upper northern Thailand and is Thailand's northernmost province. It is bordered by the Shan State of Myanmar to the north, Bokeo province of Laos to the east, Phayao to the south, Lampang to the southwest, and Chiang Mai to the west. The province is linked to Houayxay, Laos by the Fourth Thai–Lao Friendship Bridge that spans the Mekong.

The average elevation of the province is 580 metres (1,903 ft). The north of the province is part of the so-called Golden Triangle, where the borders of Thailand, Laos and Burma converge, an area which prior to the rise of agricultural production of coffee, pineapple, coconuts, and banana plantations, was unsafe because of drug smuggling across the borders. The Mekong River forms the boundary with Laos, the Mae Sai and Ruak River with Burma. Through the town of Chiang Rai itself, flows the "Mae Kok" Kok River and south of it the Lao River, a tributary of the Kok.

While the eastern part of the province is characterized by relatively flat river plains, the northern and western part consists of the hilly terrain of the Thai highlands with the Khun Tan Range and the Phi Pan Nam Range in the west and the Daen Lao Range in the north. While not the highest elevation of the province, the 1,389-metre (4,557 ft) high Doi Tung (Flag Hill) is the most important terrain feature. Wat Phra That Doi Chom Thong wat on top of the hill, according to the chronicles, dates back to the year 911. Nearby is Doi Tung Royal Villa, former residence of the late princess mother Somdej Phra Srinagarindra (mother of King Rama IX). Thanks to her activities the hills were reforested, and the hill tribes diverted from growing opium poppies to other crops including coffee, bananas, coconuts, and pineapples. The total forest area is 4,585 km² (1,770 sq mi) or 39.9 percent of provincial area.

There are eight national parks, of which seven in region 15 (Chiang Rai) and Doi Wiang Pha in region 16 (Chiang Mai), they are the protected areas in Chiang Rai province. (Visitors in fiscal year 2024)

The two wildlife sanctuaries in region 15 (Chiang Rai) are not in Chiang Rai province.

Chiang Rai was founded in 1262. Populations have dwelled in Chiang Rai since the 7th century and it became the center of the Lanna Kingdom during the 13th century. The region, rich in natural resources, was occupied by the Burmese until 1786.

Chiang Rai province's golden triangle bordering Laos and Burma was once the hub of opium production.

Chiang Rai became a province in 1910, after being part of the Lanna Kingdom for centuries. After Lanna was incorporated into Thailand, it remained an autonomous region and thus the Chiang Rai area was administered from Chiang Mai.