W21 may refer to:

• W21, a hydrogen bomb design for the US military
• Mercedes-Benz W21, six-cylinder passenger car launched in 1933
• Wanderer W21, middle-class six-cylinder sedan introduced by Auto Union's Wanderer company in 1933

• William Rhoden
• William Rhodes (cricketer)
• William Rhodes (cricketer, born 1936)
• William Rhodes (rugby league born circa-1900)
• William R. Hodgson
• William R. Hollingsworth Jr.
• William R. Howley
• William Ribeiro Soares
• William Rice (1788)
• William Rice (librarian)
• William Rice (MP)
• William Rice Mulliner
• William Rice (rower)
• William Richard Arnold
• William Richard Arnold (rugby union)
• William Richard Basham
• William Richard Bird
• William Richard Cortis
• William Richard Gowers
• William Richard Howley
• William Richard Lavender
• William Richard LeFanu
• William Richard Moore
• William Richard O'Byrne
• William Richard Randell