K31 is a Swiss rifle.
K31 may also refer to:
- K-31 (Kansas highway)
- HMS Camellia (K31), a corvette of the Royal Navy
- HSwMS Visby (K31), a corvette of the Swedish Navy
- Potassium-31, an isotope of potassium
- Soviet submarine K-31
- Sonata in B-flat, K. 31, by Wolfgang Amadeus Mozart