M99 or M-99 may refer to:
Places
- M-99 (Michigan highway), a state highway in south central Michigan
- Messier 99, an unbarred spiral galaxy approximately 60 million light-years away in the constellation Coma Berenices
- M99 road (Johannesburg), Metropolitan route in the City of Johannesburg, South Africa
Rifles
- Barrett M99, a sniper rifle
- Zijiang M99, a Chinese anti-materiel rifle
Tranquilizer
- Etorphine (a.k.a. Immobilon or M99), a veterinary tranquilizer