0th or zeroth is an ordinal for the number zero sometimes used under zero-based numbering, and can refer to:
- 0th grade, another name for kindergarten
- 0th dimension
- 0th planet, a name for Vulcan (hypothetical planet)
- 0th United States Congress, also called the Congress of the Confederation
- January 0 or January 0th, an alternate name for December 31
- The zeroth law of thermodynamics
- Zeroth-order logic
- The Zeroth Law, an addition to Isaac Asimov's Three Laws of Robotics
- Zeroth order approximation; see Orders of approximation
- Zeroth, the fastest demo or highest score on a level or episode in N (video game)