Ynysddu is a small village located in the Sirhowy valley of south-east Wales. It is part of the district of Caerphilly within the historic boundaries of Monmouthshire. Located next to Cwmfelinfach it is 4.3 miles (6.8Km) north of the town of Risca and 4 miles (5.1Km) south of the market town of Blackwood.

History
Ynysddu was founded in the early 19th century by the enlightened local colliery owner John Hodder Moggridge, who lived at nearby Woodfield Park. It got its name from Ynsyddu Farm, which was built by Moggridge in 1804. Ynysddu comes from the Welsh descriptive words 'Ynys' ('island') and 'ddu' ('black'), but Ynysddu is neither black, nor an island.

It is possible that the name simply means 'dark place' since the village is situated in a deep, narrow, valley and dominated by an overhanging mountain.

After he became alarmed at the terrible living conditions of the poor in the 1820’s Moggridge also built ‘The Ranks’. Founded as a social experiment ‘The Ranks’, like similar properties in Blackwood, allowed a small number of workers to lease a plot of land to build a cottage and garden allotment. They were situated near the Black Prince pub, but have long since disappeared

In 1901 Kelly's Directory of Monmouthshire, described the village thus:

   "YNYSDDU is a hamlet of this parish, having a station on the Sirhowy section, London and North Western railway, 10¼ miles north-west from Newport. Here are Baptist and Calvinistic and Primitive Methodist chapels. Post Office.- John Hughes, sub-postmaster. Letters received through Newport at 8.55 a.m.; dispatched, 6.30 p.m. week days only. Postal orders are issued here, but not paid. Pontllanfraith is the nearest money order & Blackwood the nearest telegraph office
   Board School, erected in 1877, at a cost of £1,500, & erected in 1899, for 167 children; average attendance, 101; Thomas Harries Phillips, master; Miss Susanna Evans, assistant mistress. Railway Station, James Williams, station master"

Present day
The electorate in 2008 was 2,905.

Ynysddu is easily accessible from all road networks as it is approximately 10 minutes from the M4 motorway and 35 minutes from the Severn Bridge.

Notable residents
The Welsh poet William Thomas (1832-1878). was born in Ynysddu, taking his bardic name Islwyn (meaning below the grove) from the parish of Mynyddislwyn, itself named after the mountain which towers over the village. His two best-known poems are both entitled "Yr Ystorm" ("The Storm"), one of which is a long philosophic poem over 9,000 lines long.

Sport and leisure
Ynysddu Rugby Club is a member of the Welsh Rugby Union and is a feeder club for the Newport Gwent Dragons. The first Ynysddu game recorded was in 1894 against Abercarn, and was played in the church field at Mynyddislwyn. Ynysddu originally played in an all-black strip, but now play in Black and Red.

Ynysddu Crusaders Football Club was formed in 2008 with the amalgamation of Ynysddu Welfare Crusaders AFC and Crusaders AFC (Seniors). They are affiliated to the Gwent County Football Association and are members of the Monnington Group South Wales Amateur Football League and the Ryan Transport Ltd North Gwent Football League. Club Colours are gold and black striped shirts, with black shorts and socks. They play their home games at Nine Mile Point Welfare Grounds.

Nearby is the spectacular scenery of the Sirhowy Valley Country Park, where the Ynys Hywel Activity Centre is located. The activity centre has a wide range of adventure activities including abseiling, canoeing, mountain biking, rock climbing, orienteering and hill walking.