Buellt or Builth was a cantref in medieval Wales, located west of the River Wye. Unlike most cantrefs, it was not part of any of the major Welsh kingdoms for most of its history, but was instead ruled by an autonomous local dynasty. During the Norman era it was associated with Rhwng Gwy a Hafren, a region independent of the Welsh monarchies and controlled by Norman Marcher Lords. In the 16th century, it was reorganized as a hundred and joined with the former kingdom of Brycheiniog to form the county of Brecknockshire.

A cantref was a medieval Welsh land division, particularly important in the administration of Welsh law.