M21 or M-21 may refer to:

Transportation[edit]

• M21 (New York City bus), a New York City Bus route in Manhattan
• M-21 highway (Michigan), a road connecting Flint and Grand Rapids
• M21 (East London), a Metropolitan Route in East London, South Africa
• M21 (Pretoria), a Metropolitan Route in Pretoria, South Africa
• M21 (Durban), a Metropolitan Route in Durban, South Africa
• Highway M21 (Ukraine)

Military[edit]

• HMS M21, Royal Navy M15 class monitor; sunk in 1918
• M21 mine, an American circular anti-tank landmine
• M21 Sniper Weapon System, a sniper rifle based on the M14 rifle
• Zastava M21, a Serbian assault rifle
• the carrier aircraft for the Lockheed D-21
• M21 Mortar Motor Carriage, a Half-Track carrying an 81mm Mortar

Other[edit]

• BMW M21, a 1983 2.4 L diesel straight-6 engine
• MSrE M-21, Hungarian aerobatics aircraft
• Lockheed M-21, an American reconnaissance drone carrier aircraft
• MacGregor 21, an American sailboat design
• Messier 21, an open star cluster in the constellation Sagittarius
• Proton Putra, car model sold in Australia as Proton M21
• M postcode area, M21 Chorlton-cum-Hardy, Firswood area of Manchester