HMM may refer to:
- Hammerton railway station, England, National Rail station code HMM.
- Heavy meromyosin, a fragment obtained from the muscle protein myosin II following limited proteolysis.
- Heavy metal music, a subgenre of rock music.
- Heroes of Might and Magic (more often referred to as HoMM), a series of turn-based strategy games by New World Computing.
- Hidden Markov model, a statistical model.
- Homenmen, an Armenian sporting organization.
- Hum (sound), a sound one makes when thinking, e.g. "Hmm, I wonder..."
- Human malignant melanoma, a tumour of melanocytic origin.
- Hyundai Merchant Marine, a South Korean logistics company.
- Marine Medium Helicopter Squadrons, United States Marine Corps squadrons that fly the CH-46 Sea Knight helicopter.