Dinieithon (Welsh for 'fort on the River Ieithon'; also known as Dineithon or Cefnllys) was a commote within the cantref of Maelienydd, in the medieval region of Rhwng Gwy a Hafren in Wales.

The southernmost of the four commotes in Maelienydd, it was also the most important due to its arable land and the presence of the region's administrative centre at Cefnllys.

Dinieithon was probably part of the Kingdom of Powys in the Early Middle Ages.

In 1093, the Norman barons Roger de Montgomery, Ranulph de Mortimer, and Philip de Braose conquered the region, and Ralph Mortimer built a motte-and-bailey at Dinieithon to secure his new territory.

[1] The native Welsh made a recovery in the area, until Ralph Mortimer II built a masonry castle at Cefnllys, which subsequently became seat of the cantref.