SIU may refer to:
- Southern Illinois University, in Illinois, United States
- Salem International University, in West Virginia, United States
- Shinawatra (International) University, in Bangkok, Thailand
- Symbiosis International University, in India
- Seafarers International Union of North America, an organization of labor unions
- Special Investigations Unit, a police oversight agency in Canada
- Siuna Airport, in Nicaragua
- Section of independent utility, a portion of highway in the US Road System
- Shake It Up, a Disney Channel TV series
Siu may also refer to:
- Xiao (surname), a demonym, from source of 4
- Shao surname, from source of 7