Marmayogi (transl. The Mysterious Sage) is a 1951 Indian Tamil-language swashbuckler film directed by K. Ramnoth and produced by M. Somasundaram. An adaptation of the novel Vengeance by Marie Corelli and William Shakespeare's play Macbeth, the film was shot simultaneously in Hindi as Ek Tha Raja (transl. Once There Was A King). It stars M. G. Ramachandar, Anjali Devi and Madhuri Devi, with S. V. Sahasranamam, Serukalathur Sama (playing the title role), N. Seetharaman, S. A. Natarajan, M. N. Nambiar and M. Pandari Bai in supporting roles.

Development of the film began after the success of Rajakumari (1947), when Ramachandran approached writer A. S. A. Sami to write a script which revolves around him. Sami wrote a script inspired by Vengeance, Macbeth, and Robin Hood. S. M. Subbaiah Naidu and C. R. Subbaraman composed all the songs in the soundtrack album. M. Masthan and W. R. Subbarao handled the film's cinematography. The film was edited by M. A. Thirumugam.

Marmayogi was released on 2 February 1951. It became a commercial success and established Ramachandran's image as a star. It was the first Tamil film to receive an A certificate (adults only) from the Central Board of Film Certification.