Adder may refer to:
Snakes:
- Any of several groups of venomous snakes of the Viperidae family, including Vipera berus, the common European adder, found in Europe and northern Asia
- Acanthophis sp., death adders, a genus of elapid snakes found in Southeast Asia and Australia
- Heterodon sp., hog-nosed snakes, a genus of harmless colubrid snakes found in North America
- Agkistrodon contortrix mokasen, the northern copperhead, a venomous viper found in the eastern United States
Other:
- Adder (Farthing Wood), a character from The Animals of Farthing Wood
- Adder (electronics), an electronic circuit designed to do addition
- AA-12 Adder, a Russian air-to-air missile
- HMS Adder, any of seven ships of the Royal Navy
- USS Adder (SS-3), an early US submarine
- Adder Technology, a KVM and A/V Manufacturing company
- Adder, a fictional commanding officer in the Advance Wars video games