Llyn Fawr is a lake in south Wales, best known as the site of an important hoard of weapons and tools from the late Bronze Age and early Iron Age.
Llyn Fawr has an area of 9.8 hectares (24 acres) and lies south of Rhigos in the unitary authority of Rhondda Cynon Taff. It lies near the source of the River Rhondda Fawr, at an altitude of 368 metres, and is overlooked by the crags of Craig y Llyn. Originally a smaller lake, it was converted into a reservoir in the early 20th century.
The Llyn Fawr hoard was discovered between 1911 and 1913 during the construction of the reservoir. It contains many objects from the late Bronze Age, but also a number of iron objects, notably an iron sword of the Hallstatt type. Two other iron implements were found, a spear and a socketed sickle. Bronze items included two cauldrons and axeheads. The items appear to have been placed in the lake as votive offerings. The date of these items is uncertain because of the context of the finds, but the sword is thought to date from about 650 BC. It is the earliest iron object discovered in Wales.
This hoard gave its name to the Llyn Fawr Phase, the last phase of the Bronze age in Britain.