Dowlais (Welsh pronunciation: [ˈdɔu̯lai̯s]) is a village and community of the county borough of Merthyr Tydfil, in Wales. The name is derived from the Welsh du meaning 'black' and glais meaning 'stream'. Dowlais is notable within Wales and Britain for its historic association with ironworking; once employing, through the Dowlais Iron Company, over 7,000 people, the works being at one stage the largest in the world.

Dowlais was originally part of the parish of Merthyr Tydfil. In 1872 the population was 15,590.

Dowlais was an electoral ward to Mid Glamorgan County Council from 1973 to 1996. At the 2011 census the electoral ward had a population of 6,926, The population of the Community was 4,270 at the 2011 census following the exclusion of Pant. Since 1996 Dowlais has been a ward to Merthyr Tydfil County Borough Council. In 2003 Dowlais was represented by an independent councillor, John Pritchard, who was also Mayor of Merthyr Tydfil.

In 1965 Welsh historian Frederick Rees documented: 'In September, 1759, a partnership of nine persons, including some local names and that of Isaac Wilkinson, was formed with a capital of £4,000 to set up a furnace or furnaces near Merthyr on the banks of the stream called the Dowlais' (Isaac Wilkinson was a pioneering English industrialist who had bought the Bersham Ironworks, near Wrexham in north-east Wales.) The partnership took out a lease from Lady Windsor, the widow of Herbert, Lord Viscount Windsor. The terms of the lease were extremely favourable; they comprised a yearly rent of £31 and without any requirement to pay royalties on the iron and coal extracted from the site, which prompted Rees to comment that 'the prospects were considered very doubtful.' In 1767, the partnership offered John Guest from Broseley in Shropshire the post of manager of the works because of his knowledge of the Coalbrookdale method of smelting iron with coal. For some years, Guest didn't make much headway. But, by 1782, when he became a partner, the ironworks were 'well established'. And, following the involvement of successive members of his family, the works became extremely successful.

By the mid-1840s there were between 5000 and 7000 men, women and children employed in the Dowlais works. During the early to mid 1800s the ironworks were operated by Sir John Josiah Guest and, from 1833, his wife Lady Charlotte Guest. Charlotte Guest introduced welfare schemes for the ironworkers. She provided for a church and a library. The school, which dates from 1819, was improved and extended, becoming "probably the most important and most progressive not only in the industrial history of South Wales, but of the whole of Britain". In the 1850s, after Sir John's death, the works existed under the control of a board of trustees. In 1865 the Bessemer steel making process was introduced to Dowlais, with £33,000 being spent on a new steelworks. And, following the involvement of successive members of his family, the works became extremely successful. As Welsh historian John Lloyd put it in 1906: 'the Dowlais Works are known to all the world, ranking as one of the largest in the United Kingdom ....'

Steel production at Dowlais eventually ceased in 1936 due to the Great Depression. However, the iron foundry continued until 1987.

Little remains of the works that once sustained the community throughout the Victorian era until the 1930s, the two notable buildings that remain are the Engine House, now used as a community centre and the stable block which is now social housing.

Dowlais House, which has now been demolished, was once home to Sir John Josiah Guest and Lady Charlotte Guest, and it was at Dowlais House that Lady Guest translated the Mabinogion. The Guest Memorial Library (1863); commissioned by Lady Guest and designed by Charles Barry, still stands.