Cathays (/kəˈteɪz/ kə-TAYZ; standardised Welsh: Cathays; sometimes Y Waun Ddyfal, 'the constant meadow') is a district and community in the centre of Cardiff, capital of Wales. It is an old suburb of Cardiff established in 1875. It is densely populated and contains many Victorian terraced houses. The area falls into the Cathays ward. It is the third most populous community in Cardiff, having a population of 18,002 in 2011.

The area that is now Cathays was formerly known in Welsh as Y Waun Ddyfal and in English as Little Heath (to distinguish it from Great Heath).

The name Cathays first appeared in 1699 as Catt Hays and originally denoted a tract of common land north-east of Cardiff, now represented by Cathays Park. The second element is a derivative of Old English haga, meaning 'park or enclosure', while the first element has been variously traced to the Welsh word cad, meaning 'battle', and the Old English word catt, meaning 'wildcat'.

By the medieval period farmland outside the old Cardiff Castle, Cathays takes in the northern limit of mediaeval Cardiff — marked by the crossroads of Fairoak Road and Crwys Road.

After John Stuart, 1st Marquess of Bute, married Charlotte Hickman-Windsor (daughter of Herbert Windsor, 2nd Viscount Windsor) on 12 November 1766, he inherited great further lands, including in Cathays to the north of his existing estate which he had partially developed. He then bought other properties and farms to the north and east, including what became Cathays Park. There he built Cathays House at a cost of £40,000 and at further cost landscaped Cathays Park. After his son took over the title he preferred to live in Cardiff Castle, so demolished the house in 1815, and turned Cathays Park into purely enclosed parkland for grazing.

Following the 2nd Marquis' development of Cardiff Docks, and the resultant number of new workers flocking to Cardiff, in 1875 the then rural Cathays became a suburb of Cardiff. At that time, a few streets led off Woodville Road and Cathays Terrace. By 1900 the urbanisation of Cathays was virtually completed. Allensbank and Wedal farms survived briefly. In 1914 they became no more than local place names.

Maindy Barracks opened in 1871. With United States Army troops temporarily stationed in transit in Cardiff during both World War I and World War II, the footpath between Gelligaer Street and New Zealand Road became known as "BURMA Road" (from the phrase "Be Undressed and Ready My Angel"), as they came to meet prostitutes.

Cathays does not have a community council.