Magnum opus (sometimes Opus magnum, plural magna opera), from the Latin meaning great work, refers to the best, most popular, or most renowned achievement of an author, artist, or composer, and most commonly one who has contributed a very large amount of material. It is another way of saying "masterpiece."

The term is also used in several spiritual traditions, such as Qabalah, Thelema, and alchemy, with a complex meaning that mainly refers to the philosopher's stone.

Magnum Opus is an album released in 1995 by Yngwie J. Malmsteen.