E121 may refer to:

• European route E121
• Citrus Red 2, a food dye, sometimes used in the United States to color the skin of oranges
• Unbiunium, a hypothetical element

• The Dublin Fusiliers
• The Dublin Gazette
• The Dublin Lockout of 1913
• The Dublin Magazine
• The Dubs
• The Duchess
• The Duchess of Chicago
• The Duchess of Duke Street
• The Duchess of Malfi
• The Duchy of Mantua
• The Duck family
• The Duck Variations
• The dude
• The Dudesons
• The Dudley Boys
• The Dudley Boyz
• The Dudley Brothers
• The Dudley Do-Right Show
• The Dudleys
• The Dudleyz
• The Duelist
• The Duelist (magazine)
• The Duellists
• The Duhks
• The Duke