J21 or J-21 may refer to:

• County Route J21 (California), a road in California
• Soko J-21 Jastreb, a Yugoslav designed sub-sonic jet aircraft
• a model of NER Class C1 steam locomotive
• HMSwS Sodermanland (J21), an Ostergotland class destroyer of the Swedish Navy
• SAAB J 21, a unique Swedish fighter aircraft
• Bronchiolitis ICD-10 code

• Lexical fluency test
• Lexical implication rule
• Lexicalist hypothesis
• Lexicalization Hypothesis
• Lexical relations
• Lexical replacement
• Lexical Set
• Lexical Sets
• Lexical stratum
• Lexical tree-adjoining grammar
• Lexical Variant Generation (software)
• Lexicase
• Lexicographically
• Lexi-comp
• Lexicon (album)
• Lexicon of Love II
• Lexicon of the Modern Arabic Language
• Lexicon (Will Young album)
• Lexi Davis
• Lexie Brown
• Lexie Elkins
• Lexie Hoffmeyer
• Lexi Elisha
• Lexie Liu
• Lexie Lou