Thangai (transl. Younger Sister) is a 1967 Indian Tamil-language action film, directed by A. C. Tirulokchandar and produced by Balaji. The film stars Sivaji Ganesan and K. R. Vijaya, Kanchana with K. Balaji, Nagesh in supporting roles. It was released on 19 May 1967. The film was remade in Telugu as Chelleli Kosam in 1968.^{[citation needed]}

Thangai was Sivaji Ganesan's first film under A. C. Tirulokchandar's direction. He was initially reluctant to accept the film due it being in the action genre and he was then known mainly for his dramatic films, but after producer Balaji assured him it would reinvent his image, he relented. The dialogues were written by Aaroor Dass.

The music was composed by M. S. Viswanathan, with lyrics by Kannadasan. When Viswanathan composed five tunes, Tirulokchandar and Balaji liked all the tunes; however they were challenged to choose only one tune for which they called a postman asking him to choose a tune which eventually became "Kettavarellam Paadalam".

Kalki appreciated the film for showcasing Ganesan in a new kind of role.