Llanishen Reservoir (Welsh: Cronfa Ddŵr Llanisien) is a Victorian reservoir in North Cardiff in South Wales. The reservoir is one of the reservoirs constructed as part of the Taff Fawr scheme for supplying water to Cardiff and was completed in 1886. It forms part of the Nant Fawr Corridor from the top of Roath Park to the countryside beyond Cyncoed.

The reservoir was used for water supply until the mid 1970s when it became redundant. It is now only used for leisure activities and for 'topping up' the adjacent Lisvane Reservoir.. Its current owners, Western Power Distribution (WPD), propose to redevelop the reservoir site, but there are objections from national governing bodies, the County Council, local residents and both local and national politicians.

In 1881, a report was commissioned on water resources within a reasonable distance of Cardiff. The report concluded that the Taff Fawr scheme would fulfil the town's requirements at the least cost and it was decided to create Llanishen Reservoir. Construction of the reservoir started in 1882 by the engineer James Simpson for the Cardiff Waterworks Company and was completed in 1886.

Coordinates: 51°31′44″N 3°11′20″W / 51.529°N 3.189°W / 51.529; -3.189

Llanishen (Welsh Llanisien llan church + Isien Saint Isan) is a district in the north of Cardiff, the capital city of Wales. Llanishen is well known as the home of the 'Tax Offices', the tallest buildings in north Cardiff and a landmark for miles around. The office complex overlooks the 'Crystal and Fishguard estates', Parc Ty Glas industrial estate, old village, leafy suburban roads and various parkland that constitute the district. Llanishen is also home to a fully featured Leisure Centre and the 60-acre (240,000 m²) Llanishen Reservoir - a local beauty spot that is connected to a green corridor which bisects the city from the rising hills in the north to Cardiff Bay in the south.

Originally wooded farm land, in A.D. 535 two monks came eastwards from the small religious settlement of Llandaff, aiming to establish new settlements, or "llans", in the land below Caerphilly mountain. With fresh water from the Nant Fawr stream, one of the monks Isan founded his llan on the site of the modern day Oval Park.