Arunachala (Aruṇācala, “red mountain”) refers to the holy hill at Tiruvannamalai  in Tamil Nadu, India. The hill is also known by the names Arunagiri, Annamalai Hill, Arunachalam, Arunai, Sonagiri and Sonachalam.

It is one of the five main shaivite holy places in South India. The Annamalaiyar Temple, a temple of Lord Shiva is located at the base of the hill.  Every year in the Tamil month of Karthigai (October–November), the Karthigai Deepam (Light) is lit atop the hill.

It is also an important place for devotees of Sri Ramana Maharshi with Sri Ramana Asramam situated at its foothills.