A siphon is a tube in an inverted U shape which enables a liquid, under the pull of gravity, to flow upwards and then downwards to discharge at a lower level.

Siphon may also refer to:

• Soda siphon, device used for dispensing carbonated water
• Siphon (insect anatomy)
• Siphon (mollusc), an organ of many aquatic molluscs through which water (or air) flows
• Oral siphon of Urochordata
• Siphon (horse), a racehorse
• Carotid siphon, a portion of the human internal carotid artery running through the cavernous sinus
• Siphon (cave), a passage in a cave that is submerged under water
• Anti-siphoning law, a term in television broadcasting
• GWR Siphon, a series of enclosed milk churn transport wagons built by the Great Western Railway