Mysore State, colloquially Old Mysore, was a political territory within the Dominion of India and the subsequent Republic of India from 1947 until 1956. The state was formed by renaming the Kingdom of Mysore, and Bangalore replaced Mysore as the state's capital. When Parliament passed the States Reorganisation Act in 1956, Mysore State was considerably enlarged when it became a linguistically homogeneous Kannada-speaking state within the Republic of India by incorporating territories from Andhra, Bombay, Coorg, Hyderabad, and Madras States, as well as other petty fiefdoms. It was subsequently renamed Karnataka in 1973.

The Kingdom of Mysore was a realm in the southern part of Deccan Plateau traditionally believed to have been founded in 1399 by two Hindu brothers, in the vicinity of the modern city of Mysore. From 1799 until 1950, it was a princely state, until 1947 in a subsidiary alliance with British India. The British took direct control over the princely state in 1831. Upon accession to the Dominion of India, it became Mysore State, later uniting with other Kannada speaking regions to form the state of Karnataka, with its ruler remaining as Rajapramukh until 1956, when he became the first governor of the reformed state.