Mullaperiyar Dam, Gravity dam in Idukki District, Kerala, India.

Mullaperiyar Dam is a gravity structure on the Periyar River in Idukki District, built using masonry within the Cardamom Hills of the Western Ghats. It extends 365 meters across the river valley and stands 53 meters above the riverbed.

The dam was built between 1887 and 1895 under Colonel John Pennycuick of the Madras Engineers, after the project was approved in 1882 and his revised estimate accepted in 1884. A lease indenture of 29 October 1886, signed by Dewan V. Ram Iyengar and J. C. Hannington, gave the British Secretary of State the Periyar works for 999 years at five rupees per acre annually, with 8,000 acres for the reservoir and 100 acres for the dam.

The name combines Mullayar and Periyar, the two rivers that meet below the dam. The reservoir, Periyar Thekkady Lake, is ringed by Periyar National Park, and the water is diverted east through a tunnel to the Vaigai basin.

The dam stands in Kerala but is operated and maintained by Tamil Nadu, so access and inspection are controlled by that state's Public Works Department. The catchment lies entirely in Kerala, and the Periyar National Park surrounds the reservoir at Thekkady. The structure is a masonry gravity dam, not a walkable visitor route.

The reservoir supplies agricultural land in Tamil Nadu despite being located in Kerala, leading to long negotiations between both states. Water flows through a tunnel carved through the mountains to reach the plains on the other side.