H21

H21 or H-21 may refer to:


 * British NVC community H21, a type of heath community in the British National Vegetation Classification
 * Highway H21 (Ukraine)
 * HMS H21, a Royal Navy H-class submarine
 * HMS Scimitar (H21), a Royal Navy S-class destroyer
 * Mata Hari (1876–1917), German spy code named H-21
 * Piasecki H-21, an American helicopter
 * Institute H21, a Czech think tank that created the website Prezident 21