A Cantref (Welsh pronunciation: [ˈkaːntrɛ(v)]) was a medieval Welsh land division, particularly important in the administration of Welsh law.

Land in medieval Wales was divided into cantrefi, which were themselves divided into smaller cymydau (commotes). The name "cantref" is derived from "Cant" ("a hundred") and "tref" ("town" in modern Welsh but formerly used for much smaller settlements). The cantref is thought to be the original unit, with the commotes being a later division. Cantrefi could vary considerably in size; most were divided into two or three commotes but the largest, the "Cantref Mawr" or "Great Cantref" in Ystrad Tywi (now in Carmarthenshire) was divided into seven commotes. To give an idea of the size of a cantref, the island of Anglesey was divided into three cantrefi, Cemais, Aberffraw and Rhosyr.

The antiquity of the cantrefi is demonstrated by the fact that they often mark the boundary between dialects. Some were originally kingdoms in their own right, others may have been artificial units created later.