Siwan may refer to
- Sivan, a month of the Hebrew calendar
- Siwan (Mandaean month), a month of the Mandaean calendar
- The Welsh form of Joan (first name)
- Siwan or Joan, Lady of Wales, the wife and consort of Llywelyn the Great, Prince of Wales
- Siwan (play), by Saunders Lewis
- Sewant, beads used as a currency by Native Americans
- Siwan (album), a musical orchestra project by Jon Balke, and featuring Amina Alaoui
- Siwan, Henan, a town in Xichuan County, Henan, China