M21 or M-21 may refer to:
Transportation M21 (New York City bus), a New York City Bus route in Manhattan M-21 (Michigan highway), 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 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 Lockheed M-21, a variant of the Lockheed A-12 and the carrier aircraft for the Lockheed D-21 M21 mortar motor carriage , a half-track carrying an 81 mm mortar
Other 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 M21, a postcode district for an area of south Manchester, see M postcode area McLaren M21, a Formula Two racing car