Defiant could refer to:

• One of the various Federation starships of that name:
  • USS Defiant (NCC-1764), a Constitution-class starship
  • USS Defiant (2370), lead ship of the class, the first Defiant-class starship to bear the name
  • USS Defiant (2375), the second Defiant-class starship to bear the name (formerly the USS Sao Paulo)
• The ISS Defiant, a Terran Rebellion starship from the 24th century mirror universe
• HMS Defiant, The British Royal Navy ship
• The Defiant-class escort, a Federation starship type
• "Defiant", a Season Three episode of Star Trek: Deep Space Nine