1D, 1-D, or 1d can refer to:
- Alpha-1D adrenergic receptor
- Astra 1D, a satellite
- Canon EOS-1D, Canon's first professional digital camera
- Long March 1D, a satellite
- One-dimensional space in physics and mathematics
- One Direction, an English-Irish boy band
- Penny (British pre-decimal coin), routinely abbreviated 1d.
1D, the hexadecimal code for the Group Separator control character