U21

U21 or U-21 may refer to:

Naval vessels

 * GERMAN SUBMARINE U-21, various vessels
 * INS Jamuna (U21), a sloop of the Indian Navy
 * SM U-21, a submarine of the Austro-Hungarian Navy

Other uses

 * Beechcraft U-21 Ute, an American utility aircraft
 * Great rhombihexahedron
 * Small nucleolar RNA SNORD21
 * Universitas 21, an international network of research-intensive universities
 * Uppland Runic Inscription 21