31 Squadron
- 31st Airlift Squadron
- 31st Battalion (disambiguation)
- 31st Brigade (disambiguation)
- 31st Combat Communications Squadron
- 31st Division (disambiguation)
- 31st Group (disambiguation)
- 31st Reconnaissance Squadron (disambiguation)
- 31st Regiment (disambiguation)
- 31st Rescue Squadron
- 31st Squadron (Belgium)
- 31st Tactical Training Squadron
- 31st Test and Evaluation Squadron
- 31st Training Squadron (JASDF)
- Marine Aviation Logistics Squadron 31
- No. 31 Squadron (Finland)
- No. 31 Squadron RAAF
- No. 31 Squadron RAF
- VFA-31