Llanddwyn Island (Ynys Llanddwyn in Welsh) is a small peninsula/tidal island on the south coast of the Isle of Anglesey in the Welsh Snowdonia region, measuring only 0,3 square kilometres. Although small, it is quite important in many ways, both in the area of natural history and in the area of geological heritage, but it has an important maritime past as well.

Landdwyn Island is situated near the southern entrance to the Menai Strait and became important with the increase of slate shipping from the nearby ports of Bangor and Caernarfon. The small lighthouse in my picture, called Tŵr Bach (Little Tower), was built at the tip of the island to provide guidance to ships heading for the Strait. No exact construction date is recorded. Later on, around 1845, a bigger lighthouse, Tŵr Mawr (Big Tower) was built in the island. It is nowadays out of service. The small lighthouse, however, regained its function in 1975 when a new, modern navigation beacon was installed on top of it.

Llanddwyn Island is rich in geological heritage. It has well-preserved exposures of late Neoproterozoic-Cambrian mélange, which can easily be seen from and even on the beaches around the island. The island is also very important for birds, who feed in the tidal creeks off the beaches. In spring, it provides a breeding ground for countless birds.

Today, the island is uninhabited. That used to be different, given that there is a row of small cottages, known as the Pilot's Cottages, on the island. The island is also associated with Saint Dwynwen. The name Llanddwyn means 'The church of St. Dwynwen'. Dwynwen is the Welsh patron saint of lovers, the Welsh equivalent of St. Valentine. Her Saint's Day is January 25.