M. S. Viswanathan (b. June 24, 1928) (or M.S.V.), is a popular south Indian music director for five decades. He is popularly known as "Mellisai Mannar" (Tamil, for "The King of Light Music"). His major works are in Tamil and Telugu films. He has also acted and sung in a few films.

Viswanathan was born to Subramanian and Narayani Kutti (or Naanikutii), in Manayangath house (from which one of his initials is derived) in Ilappulli village in Kerala, India in 1928. He lost his father when he was four. He was saved by his grandfather when his mother decided to kill him and herself as an escape from abject poverty and lack of support. He sold foodstuff in a movie theater during his childhood and later learned music from Neelakanda Bhagavathar. He gave his first stage performance stage in Trivandrum, when he was 13 years old.

Of Malayalee origin, he settled as a prolific film music composer in the Tamil film industry. His first score is for the film Panam (1952). He dominated film music composing in the South Indian film industry during the 1960s and 1970s and composed scores for over 1000 films. His achievements range from introducing fresher melodies, style and orchestration approaches to incorporating various genres of what is now called world music into Indian cinema music. He sung over 500 of his own songs, as well as those of other music directors like V. Kumar, Shankar Ganesh, Ilayaraaja, Gangai Amaran, Deva and A. R. Rahman. He also scored music for many non-film albums and for political campaigns. He also acted in many films and television series.

He often worked with the well-known Tamil poet and lyricist Kannadasan. In recent times, Viswanathan has focused on composing devotional music.