The River Dun, also known as the Glendun River (in Irish: Abhann Duinne, brown river) is a river in County Antrim, Northern Ireland. It runs through Glendun, one of the nine Glens of Antrim. The river is named after its brown colour, which comes from the peat bogs at its source on the slopes of Slievenanee on the Antrim Plateau. The source of the river is a few hundred metres from that of the River Bush, which flows north-east to meet the sea at Bushmills. Cushendun (Bun Abhann Duinne, "foot of the River Dun" in Irish – although the English name derives from Cois Abhann Duinne, "beside the River Dun") is a small coastal resort town lying at the mouth of the River Dun and Glendun.
The river is spanned by a three arched viaduct completed by Charles Lanyon in 1839. Glendun Viaduct, known locally as the Big Bridge, spans the river in the centre of the glen. It was built between 1834 and 1839, though no work was done during the winter months. The stones were quarried in Layde, taken by boat to Cushendun and then transported by cart to the bridge site.
The river supports a population of brown trout which rarely attain weights exceeding one pound due to the naturally acidic chemistry of the water. Sea trout and salmon enter the river and make their way to the upstream spawning beds from July onwards. Eels are also present.
Coordinates: 55°07′30″N 6°02′45″W / 55.12487°N 6.04574°W