Lupin III: The Mystery of Mamo, also known as Lupin III: The Secret of Mamo, is a 1978 Japanese animated science fiction action adventure comedy film. It is the first animated feature film based on the 1967–69 manga series Lupin III by Monkey Punch. The film was originally released in Japan as Lupin III (ルパン三世, Rupan Sansei), but was later retitled to Lupin III: Lupin vs. the Clone (ルパン三世 ルパンVS複製人間(クローン), Rupan Sansei Rupan tai Kurōn). Directed by Sōji Yoshikawa, who co-wrote the screenplay with Atsushi Yamatoya, the film was produced by animation studio Tokyo Movie Shinsha and distributed by Toho. The film's plot follows master thief Lupin III, who attempts to foil Mamo ― a wealthy and powerful recluse seeking immortality ― while trying to win the affections of his rival and would-be lover, Fujiko Mine.

The film was theatrically released in Japan on December 16, 1978, by Toho. Since its original Japanese release, the film has been licensed to several companies for release in North America and Europe, with four different English dubs of the film being produced in that time. In 2013, North American distributor Discotek Media released the film on a DVD containing all four English dubs, as well as extensive liner notes and essays about the film and its production. In 2022, Discotek Media released the film on Blu-ray.

Financially successful upon release, The Mystery of Mamo has continued to polarize critics and fans, especially in English-speaking markets. Praise for the film tends to be aimed at its originality and faithfulness to the manga, and criticism usually focuses on the execution of the third act. The English dubs, though varying widely in terms of their production quality, interpretation of the dialogue and voice acting, have frequently been singled out for praise.