Lao or Laotian (ພາສາລາວ, BGN/PCGN: phasa lao, IPA: [pʰáːsǎː láːw]) is a tonal language of the Tai–Kadai language family. It is the official language of Laos, and also spoken in the northeast of Thailand, where it is usually referred to as the Isan language. Being the primary language of the Lao people, Lao is also an important second language for the multitude of ethnic groups in Laos and in Isan. Lao, like all languages in Laos, is written in an abugida script. Although there is no official standard, the Vientiane dialect has become the de facto standard.

The Lao language is descended from Tai languages spoken in what is now southern China and northern Vietnam (probably by some of the various peoples referred to as the Baiyue) in areas believed to be the homeland of the language family and where several related languages are still spoken by scattered minority groups. Due to Han Chinese expansion, Mongol invasion pressures, and a search for lands more suitable for wet-rice cultivation, the Tai peoples moved south towards India, down the Mekong River valley, and as far south as the Malay Peninsula. Oral history of the migrations is preserved in the legends of Khun Borom. Tai speakers in what is now Laos pushed out or absorbed earlier groups of Mon–Khmer and Austronesian languages.