Isan also written as Isaan, Isarn, Issan, or Esarn) is the northeast region of Thailand. It is located on the Khorat Plateau, bordered by the Mekong River (along the border with Laos) to the north and east, by Cambodia to the southeast and the Prachinburi mountains south of Nakhon Ratchasima. To the west it is separated from Northern and Central Thailand by the Phetchabun mountain range.

Agriculture is the main economic activity, but due to the socio-economic conditions and hot, dry climate output lags behind that of other parts of the country. This is Thailand's poorest region.

The main language of the region is Isan, which is similar to Laotian. Considered by some to be a dialect of Laotian, but written in the Thai alphabet, Isan is among the Chiang Seng and Lao-Phutai languages, which are members of the Tai languages of the Tai-Kadai language family. Thai is also spoken, with regional accents, by almost everyone. Khmer (the language of Cambodia) is widely spoken in regions near the Cambodian border (Buriram, Surin, and Sisaket). Most of the population is of Lao origin, but the region's incorporation into the modern Thai state has been largely successful.

Prominent aspects of the culture of Isan include the indigenous form of folk music, called mor lam, Muay Thai boxing, cock fighting, and celebratory processions. Isan food, in which sticky rice (khao niao) and chillies are prominent, is sufficiently distinct from Thai cuisine that it is considered unique. Sticky rice is a staple of Thai Northeastern cuisine, and it accompanies meals frequently.