Magnus was an American maker of inexpensive reed instruments founded in 1944 in New Jersey by Danish immigrant Finn Magnus. Originally incorporated as the International Plastic Harmonica Corporation, the company pioneered the use of molded styrene-plastic reed combs in place of the metal reeds traditionally used in harmonicas and free-reed instruments. The plastic construction lowered manufacturing costs and produced a softer, more mellow tone that became the brand's sonic signature.

After Finn Magnus partnered with Eugene Tracey in 1958, the renamed Magnus Organ Corporation became best known for its electric chord organs, which were marketed to children, beginners, and home hobbyists and sold in the millions through the 1960s and 1970s. Models such as the Magnus Chord Organ featured a small keyboard paired with major and minor chord buttons, with air pushed through plastic reeds by an electric blower. The company's catalog also included plastic-reed harmonicas, accordions, bagpipes, and small mechanical reed organs, all manufactured at a large factory in Linden, New Jersey.

Production wound down in the late 1970s after Finn Magnus and Tracey retired, and the sheet music catalog was eventually acquired by the publishers of World Book Encyclopedia. Magnus instruments are no longer manufactured, but the chord organs in particular have found a second life among lo-fi and experimental musicians who prize their wheezy, accordion-like timbre, and the original units circulate widely on the vintage market.

Magnus has 5 products cataloged on Equipboard, including Organs and Synth Plugins. Their gear is featured by 5 artists, with the strongest followings in Rock, Pop, and Electronic. Notable users include David Bowie, Julian Koster, and Isaiah Ikey Owens.