Porthdinllaen (English Porth Dinllaen), is a small coastal village in the Dwyfor locality on the Llŷn Peninsula within Gwynedd, North Wales, previously in Caernarfonshire.

It has been owned by the National Trust since 1994. With views across to Yr Eifl and Snowdonia, Porthdinllaen, with Nefyn and Morfa Nefyn, form a magnificent two miles (3 km) of sweeping bay. There are only about two dozen buildings at Porthdinllaen, with the Ty Coch pub the centre of the village.

Vehicular access to the village is restricted to residents with a car permit; visitors must walk across the beach from Morfa Nefyn or across the golf course on top of the headland, past the Iron Age hillfort.

Port
Porthdinllaen was originally a fishing port, based around a natural harbour over a mile and a quarter across, and with over one hundred acres of safe anchorage. The harbour is sheltered from all but a north-easterly wind, and as the only such haven on the Llyn Peninsula, it has been used for many centuries of trading, and a place to run to for shelter in a storm.

In May 1806, a parliamentary bill approved new buildings when it seemed that Porthdinllaen would be chosen over Holyhead, Anglesey, as the route to Northern Ireland. Preferred by Isambard Kingdom Brunel as his point of embarkation for his railway, the location was further west and less accessible than Thomas Telford's road developments to Holyhead. Porthdinllaen Harbour Company was formed in 1808 in preparation by the Jones Parry family of the Madryn estate (the companies assets included the village and the harbour), but the bill before Parliament to constitute Porthdinllaen as a harbour for Irish trade was rejected in 1810.

Pig farming was important to the economy of the Llyn Peninsula, and Porthdinllaen was the main point of export to Liverpool. In 1830, the farmers and merchants asked the Madryn estate to build a bigger pier, but they estate refused. The first steamer the “Vale of Clwyd” didn't therefore enter service until 1832. Apart from goods to support the population and the farmers, Porthdinllaen imported large quantities of salt to create the Nefyn herring.

Railway
Various proposals were made to build railways to serve the harbour at Porthdinllaen, although none were eventually constructed:

• 1808 - Isambard Kingdom Brunel, as point of embarkation to Ireland
• 1830 - Samuel Holland, a slate quarry owner at Rhiw, joined Henry Archer, a businessman from Dublin, to promote the Ffestiniog Railway, incorporated by Act of Parliament on 23 May 1832
• 1845 - The Worcester and Porth-Dynllaen Railway informed the Rev. T. Parry Jones Parry of an application to Parliament of a railway track from Worcester to Porthdinllaen
• 1845 - The North Wales Railway Co. planned to construct 28 miles (45 km) of track from Bangor to Porthdinllaen
• 1860 - The Aberystwyth and Welsh Coast Railway Company wanted to construct a line from Aberystwyth to Porthdinllaen
• 1877 - The Cambrian Railways Act revived the powers conferred by the Aberystwyth and Welsh Coast Railway Act 1862 for a railway to Porthdinlleyn Harbour

Lifeboat station
In the 19th century, North Wales lacked easily passable roads, and so the sea was the easiest way to access many of the communities. Porthdinllaen, on the northern coast of the Llyn peninsula, with its sheltered east facing bay, became important as a harbour of refuge and a busy port, with over 700 ships passing through the port in 1861. After storms in 1863, the local parish priest wrote to the Royal National Lifeboat Institution to request a lifeboat be positioned in the harbour.

The boat shed and slip commissioned in 1864. Manned constantly since, Porthdinllaen lifeboat station is the only lifeboat station where Welsh is the normally spoken language of the crew. The current coxswain is Mike Davies, who has served as coxswain since 2004. The current Tyne class lifeboat is called "Hetty Rampton," and she has been in service since April 27, 1987.

Film location
Due to its highly preserved and yet maintained status, Porthdinllaen regularly acts as a film and television shooting location. In September 2004 it posed as a Scottish fishing village for the Demi Moore romantic thriller Half Light.