Kilnathur, also known as Kizh-arunai, is a town panchayat and suburb in Tiruvannamalai district, Tamil Nadu, India. It is one of the oldest settlements in the North Arcot region. It developed along with Tiruvannamalai.
As of 2001, Kilnathur had a population of 90,000. In the census, it had a population of 99,600.