Chiang Rai lies on the flat alluvial plain of the Kok River, a tributary of the Mekong, between the Daen Lao Range in the north and the Phi Pan Nam Range in the south. The Kok River runs along Chiang Rai's north side, flowing eastwards out of Burma at Tha Ton(ท่าตอม) town, bending north-eastwards and joining the Mekong River about 40 kilometres (25 mi) north-east of the city. The Lao River, a tributary of the Kok, flows south of Chiang Rai.
There are four bridges spanning the Kok River within the town's boundaries, each running south–north. Most of the terrain surrounding Chiang Rai town is either flat or has moderate hills. The exception is outward in the west and north-west directions, where limestone hills are evident, some of which have vertical exposed cliffs. That is also the direction where most of the region's hill tribe people have their villages, further afield.
The city is 860 kilometres (530 mi) north of Bangkok, about 200 kilometres (120 mi) north-east of Chiang Mai, 62 kilometres (39 mi) south of Mae Sai and the Burmese border; 60 kilometres (37 mi) south-west of the town of Chiang Saen on the Mekong River across from Laos; and 90 kilometres (56 mi) north of Phayao town. The Golden Triangle, the tripoint of the Thailand, Laos and Myanmar borders, is 55 kilometres (34 mi) north-east of the city.
Chiang Rai has a tropical wet and dry climate (Köppen climate classification Aw). Winters are fairly dry and warm. Temperatures rise until April, which is hot with the average daily maximum at 34.5 °C (94.1 °F). The monsoon season runs from late April through October, with heavy rain and somewhat cooler temperatures during the day, although nights remain warm.