66th Regiment
- 66th (Berkshire) Regiment of Foot
- 66th (Leeds Rifles) (West Yorkshire Regiment) Heavy Anti-Aircraft Regiment, Royal Artillery
- 66th Armor Regiment
- 66th Armoured Regiment (India)
- 66th Division (disambiguation)
- 66th Illinois Infantry Regiment
- 66th Indiana Infantry Regiment
- 66th Infantry Regiment (Imperial Japanese Army)
- 66th Infantry Regiment (United States)
- 66th New York Infantry Regiment
- 66th Ohio Infantry Regiment
- 66th Punjabis
- United States Army Forces in the Philippines – Northern Luzon