The Castlecomer Plateau is an upland area in the North of County Kilkenny, in Ireland, extending into County Laois and County Carlow at its northern edge. The plateau is bounded on the east and south by the N9 National primary road, on the west by the N77. The River Nore flows southward past the plateau's western flank, the River Barrow past its eastern flank and it is dissected by the River Dinnin, which flows through Castlecomer itself.
The eastern section, which is the larger and more elevated part of the plateau is known as the Rossmore Plateau, after the village of Rossmore, County Carlow, which is situated at the northeastern extremity near the highest point on the plateau (334 m) and is just 5 km from Carlow Town situated below in the valley of the River Barrow.
The topography and soils of the area has long influenced its history. The plateau has remained sparsely populated but has also provided a refuge for the dispossessed. One of Ireland's few commercial coal deposits occurred in this upland area which has meant that it is one of the few places in Ireland that has a coal-mining history.
The area is difficult for agriculture with lower temperatures, more exposure and heavier, wetter soils than the surrounding lowlands. However it has proved excellent for forestry and there are extensive plantations of Sitka spruce at the higher elevations.