Nannau (Welsh for 'the place of many streams'') refers to both an ancient estate and its current Georgian mansion near the village of Llanfachreth, Gwynedd, North Wales. The mansion was initially inhabited by the Welsh Nanney (Nannau) family, who were direct descendants of the Kings of Powys. For over 900 years, the Nannau estate was in possession of the same family. At its largest Nannau was 12,000 acres.

Nannau is a Grade II* listed building and its parkland is listed, also at Grade II*, on the Cadw/ICOMOS Register of Parks and Gardens of Special Historic Interest in Wales. The family dynasty was founded by Madog ap Cadwgan, 1st Lord of Nannau as a cadet branch of the House of Mathrafal. The founder was a son of Prince Cadwgan ap Bleddyn (1060–1111) from the Kingdom of Powys, within what is now the Snowdonia (Eryri) national park in North West Wales. The Lord of Nannau title continued for four centuries, until the division of the cadet branches. The estate was then passed on to an heiress, Janet, who married into the Vaughan family of Hengwrt in 1719. In 1795, their descendants, the Vaughan baronets, replaced the then 17th-century mansion with a new house co-designed by Joseph Bromfield, which still stands today.

The head of the family represented the local county as Sheriff of Merionethshire and held the position nine times in 400 years between the 16th and 20th centuries. In 1911, as recorded by Encyclopædia Britannica, the families of county rank in the neighbourhood of Dolgellau included those of Nannau, Hengwrt (the famous Hengwrt Welsh MSS), Caerynwch, Fronwnion, Bron-y-gadair, Brynygwin, Brynadda, Abergwynnant, Garthangharad.

By the mid-20th century, the estate was "wrecked", and a succession of short-term owners saw much of the land sold off, the demolition of some of the 18th-century mansion, and failed attempts to establish a hotel at the hall. By 2020, the lead from the roof had been stolen, and the house was "deteriorating rapidly". Nannau is considered in the top at risk buildings by multiple organizations, including SAVE Britains Heritage and The Georgian Group. Despite acknowledgement years ago by Snowdonia Park Authority and Cadw of the critical need for intervention no action has yet been taken and the roof continues to be uncovered. The photos from August 2024 also show the roof does not have a temporary cover.

The Nanney family of Dolgellau are direct descendants of a royal dynasty, the House of Mathrafal and their Princes of Powys through Cadwgan ap Bleddyn, second son of Bleddyn ap Cynfyn (c. 11th century). Cadwgan's son Madog ap Cadwgan became the 1st Lord of Nannau. The 2nd Lord, Cadwgan ap Madog, was knighted by King Henry I of England in the Duchy of Normandy and married Gwenllian, a daughter of Owain Gwynedd (King of Gwynedd c.1137). The title of Lord was passed on from father to son until the early 16th century. Following the creation of the title Lord of Nannau, a prominent member of the family who became the 5th Lord was known as Ynyr Hen ("old Ynyr"). His son Ynyr Fychan ("Little Ynyr") sided with the English crown during the Conquest of Wales by Edward I and was rewarded for the capture of Madog ap Llywelyn (1295) during the revolt against the new administration. King Edward I also rewarded those who pledged allegiance by allowing them to keep their land. This marked the beginning of the Nannau estate dynasty. The effigy of the 7th Lord, Meurig ap Ynyr Fychan (c. 14th century) is on display at St. Mary's Church, Dolgellau. Another son of Ynyr Fychan, Einion, later became the Bishop of St. Asaph.

The Lordship of Nannau was passed on to Hywel Sele (9th Lord of Nannau, d. 1402), probably the most famous Nannau owner, who was noted for his attempted assassination of Owain Glyndŵr on the Nannau estate, before Owain set the Nannau house ablaze. Following this incident, the house was rebuilt. This would be one of at least five reconstructions during the Nannau family's tenure of the estate.

The Nannau family established a new dynasty through marriage which connected it to Tal-y-bont, Dyffryn Ardudwy, and enabled it to extend its reach throughout Wales and beyond. Lands were bought in the areas of Brithdir, Dyffryndan, and Cefnyrywen, and Dolgleder, Garthgynfor and Garthmaelan in the surrounding areas. The family established many cadet branches, beginning with Sele's brother. At the end of the 16th century, the families of Caerynwch and Cefndeuddwr emerged, and later on the family of Maes-y-Pandy. There was also an alliance with the Dolau-gwyn family.

Between 1400 and 1600, the Nannau farmlands were vastly expanded, and successive Nannau lords held government posts in and around Dolgellau. A cousin of the 10th Lord sided with the House of Tudor during the Wars of the Roses (1455–1487) and was the commander of Harlech Castle during its siege. He also fought in the Hundred Years' War in the Kingdom of France. The family helped establish Cymer Abbey, Llanelltyd, Dolgellau, and other churches in North Wales that still stand today. An heiress of the Nannau Estate was Alice, who married a descendant of Hywel Coetmor in the 15th century. She was the sole heiress of Hywel ap Meurig of Nannau. The Lord of Nannau title endured until the 13th Lord, who was the last to hold the royal title after 400 years of father/son inheritance. This period marked the beginning of surnames in Wales, and the Nanney family name emerged in the early 16th century. This coincided with a new era of Welsh Hall Houses: the Snowdonia type, in particular.