Diadem may refer to:
- Diadem (personal wear), a type of crownMilitary
- HMS Diadem was the lead ship of the Diadem-class of protected cruiser in the Royal Navy launched in 1896
- HMS Diadem was a Dido-class cruiser of the Royal Navy in World War II, launched in 1942, and transferred to Pakistan in 1956 as Babur.
- Operation Diadem, a military operation in World War II's Italian CampaignArts and entertainment
- Diadem (book series), a series of books by John Peel
- The Diadem Saga is a science fiction series by author Jo Clayton
- "Diadems," a song by the metal band, Megadeth found on the compilation albums Hidden Treasures and WarchestAnimals
- Diademed monkey, another name for Blue monkey or Pluto monkey (Cercopithecus mitis)
- Diadem Roundleaf Bat (Hipposideros diadema)
- Mograbin Diadem Snake (Spalerosophis dolichospilus)
- Diadem, Hypolimnas, one of several tropical butterflies
- Diadem spider, the European garden spider (Araneus diadematus), so called from its crown-shaped markingsOther
- Diadem (star) is the brightest star in the constellation Coma Berenices.
- DIAdem, a technical software for managing, analyzing, and reporting technical data from National Instruments