Castor most commonly refers to:

• Castor (star), a star in the Gemini constellation
• Castor, one of the Dioscuri/Gemini twins Castor and Pollux in Greco-Roman mythology

Castor or CASTOR may also refer to:

• Castor (rocket stage), a family of solid-fuel rocket stages
• Castor (software), data binding framework for Java
• CASTOR (nuclear waste), cask for storage and transport of radioactive material
• CASTOR experiment, "Centauro and Strange Object Research" at CERN
• CASTOR (spacecraft), proposed space telescope
• Castor sugar, fine sugar
• Caster (or Castor), an undriven wheel
• Caster (or Castor) angle, relevant to a steered wheel

• Castor oil plant, plant from which castor bean grows
  • Castor oil, oil of the castor bean
  • Castor wax, produced from castor oil
• Castor, a genus name of the beaver
  • Castoreum, natural scent derived from the beaver

• Castor of Rhodes, Greek grammarian and rhetorician
• Drusus the Younger (13 BC – 23 CE), Roman consul, son of the Emperor Tiberius, nicknamed "Castor"
• Antonius Castor, ancient botanist
• Saint Castor, the name of several Christian saints
  • Castor of Apt (died c. 420), French bishop of Apt
  • Castor of Karden (died c. 400), German priest and hermit

• Castor Cantero (1918–?), Paraguayan football player
• Castor McCord (1907–1963), American jazz saxophonist
• Castor (footballer) (born 1979), Edmilson Ferreira, Brazilian footballer

• Betty Castor (born 1941), American educator and politician
• Brian Castor (1889–1979), English cricketer
• Bruce Castor (born 1961), American lawyer and politician
• George A. Castor (1855–1906), American politician
• Helen Castor (born 1968), English historian
• Jean-Victor Castor (born 1962), French politician
• Jimmy Castor (1940–2012), American musician
• Kathy Castor (born 1966), American politician
• Stacey Castor (1968–2016), American murderer
• Steve Castor (born 1972/1973), American lawyer

• Castor Oyl, fictional character in the Popeye cartoons
• Castor Troy, fictional character in the 1997 movie Face/Off, portrayed by Nicolas Cage and John Travolta
• Castor, fictional character in the video game Spartan: Total Warrior
• Lord Castor, fictional character in the 2007 movie The Harpy
• Castor, fictional character in Tron: Legacy, portrayed by Michael Sheen
• "Castor, Gobernador" fictional character in The Courts of the Morning by John Buchan
• "Castor Wilds" fictional location in The Legend of Zelda: The Minish Cap by Nintendo
• "Castor", fictional persona in the video game Persona 3. Castor is used by one of the playable characters, Shinjiro Aragaki

• Castor, Alberta, Canada
• Queue de Castor River, Eeyou Istchee Baie-James, Jamésie, Nord-du-Québec, Canada
• Rivière aux Castors Noirs, river in Quebec, Canada
• Castor River (Ontario), Ontario, Canada
• Castor, Louisiana, Louisiana, United States
• Castor Creek (Little River tributary), Louisiana, United States
• Castor River (Missouri), United-States
• Castor (mountain), in the Pennine Alps on the border between Switzerland and Italy
• Castor Bay, Auckland, New Zealand
• Castor, Cambridgeshire, England
• Castor Temple, a summit in the Grand Canyon, United States
• Castor, an old name for Caistor St. Edmund, Norfolk, England
• Castor (crater) on Saturn's small moon Janus

• Castor (band), American rock band
• Castor (cloth), a woollen fabric
• Castor (trucks), a brand used by Swedish truck manufacturers Tidaholm in the 1910s
• Castor Cracking Group, Swedish music group
• HMS Castor, the name of more than one ship of the British Royal Navy
• Castor (1782 ship), a vessel launched in 1782 that made one voyage to Bengal for the British East India Company and that was last listed in 1808.
• Castor gloves, gloves made from the skin of lambs or deer
• Castor hat
• Project Castor, a set of male clones in the TV series Orphan Black
• Castore (brand), British sportswear and athletic clothing company
• Castor (1917 ship), 1917 Polish tugboat

• Caistor, a town and civil parish situated in the West Lindsey district of Lincolnshire, England
• Caster (disambiguation)
• Castro (disambiguation)
• Gastor (disambiguation)
• Kastor (disambiguation)