Pen y Fan () is the highest peak in South Wales, situated in Brecon Beacons National Park (Bannau Brycheiniog). At above sea-level, it is also the highest British peak south of Cadair Idris in Snowdonia. It is the highest point (county top) of the historic county of Brecknockshire (though in modern administrative terms, it now lies within the unitary council area of Powys). The twin summits of Pen y Fan and Corn Du at were formerly referred to as Cadair Arthur or 'Arthur's Seat'.
The mountain and surrounding area are owned by the National Trust whose work parties attempt to combat the erosion caused by the popularity of this peak with walkers. The mountain is used by the military as part of the selection process of the UK's Special Forces personnel. Three soldiers who died after collapsing in the field in July 2013 were taking part in a test march called Point-to-Point while undergoing Special Air Service selection.
The name Pen y Fan consists of the Welsh words pen ('top, head, peak, summit', etc.), y ('the') and fan, a mutated form of ban ('summit, crest, peak, beacon, hill, mountain', etc.). 'The mountain's peak' or 'the top of the summit' are both possible translations. The noun ban, in its plural form bannau, is found in the Welsh name for the Brecon Beacons: Bannau Brycheiniog.
In earlier times, Pen y Fan itself was sometimes called the '(Brecknock) Beacon' and the early modern English cartographer, John Speed, designated it as Monuchdenny. In the General View of the Agriculture of the County of Brecknock (1794), John Clark of Builth (steward to Viscount Hereford) refers to 'the VANN, or Brecknock Beacon, the undisputed sovereign of all the mountains in South Wales'. On the tithe map for Cantref parish (1839), the peak of Pen y Fan is called 'Beacon'. This usage is no longer current.
The peak is a major objective in the Beacons Way, a long distance footpath which passes from east to west across the massif, and is open to all walkers. There are well-made, evenly graded footpaths from the Storey Arms and Pont ar Daf on the A470, which is below. This and other paths on the mountain are the subject of ongoing repair and maintenance by the National Trust to limit the erosion caused by the passage of hundreds of thousands of walkers each year. There is also a major path from Cribyn, a close-by peak to the east.
The summit is on an undulating ridge stretching from Talybont Reservoir in the east, to the A470. About to the south-west lies the subsidiary top of Corn Du, beyond which the terrain drops at a moderate angle to the subsidiary top of Y Gyrn, then more steeply to the Storey Arms on the A470. To the east, the ridge drops steeply to the col connecting it to Cribyn, the next mountain along the ridge. From Corn Du, a ridge with an east-facing escarpment descends gently south towards Merthyr Tydfil.
Like much of upland Wales, Pen y Fan has a Subpolar Oceanic climate, with snowfall possible between November and May. The amount of snow and duration varies significantly from year to year. After a heavy snowfall, the mountain is a popular destination with local skiing enthusiasts.
The summit is marked by a reconstructed Bronze Age cairn with a central stone cist, similar to that on the nearby summit of Corn Du. The grave is fitted with a series of concentric stone kerbs to protect the central mound from slippage. The cist is a box formed by vertical stone slabs near the centre of the barrow, and it is currently occupied by the National Trust sign, but will have originally held the ashes or other remnants of a dead person or persons since multiple burials together are common in the British Bronze Age. It also held grave goods left with the human remains, such as flint tools, cinerary urns, or flower tributes. The similar round barrow on Fan Foel was excavated in 2002-4 and revealed such items in the central cist, the flowers being those of meadowsweet (Filipendula ulmaria).