Tŵr Mawr lighthouse (meaning "great tower" in Welsh), on Ynys Llanddwyn on Anglesey, Wales, marks the western entrance to the Menai Strait.

The lantern, originally comprising six Argand lamps burning colzo oil and fitted with catoptric reflectors, was made by De Ville & Co, London.

It projected a fixed red beam from a window on the ground floor of the attached "lean-to" building.

[3][4][5] A smaller, conical tower, with a domed top, is located to the south-east, and may be an earlier structure.

Neither tower is shown on the chart of Lewis Morris, dated 1801,[6] but they both appear on the Ordnance Survey 1818-1823 2 inches/mile map.