SC21 or SC-21 may refer to:
- SC21 (United States), a United States Navy program of the 1990s for new ship designs
- USS SC-21, a United States Navy submarine chaser commissioned in 1917 and sold in 1921
- South Carolina Highway 21
- Scandium, chemical element, symbol Sc and atomic number 21
This disambiguation page lists articles associated with the same title formed as a letter–number combination.
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