YY may refer to:
- YY, the call sign prefix for radio stations in Venezuela
- Peptide YY 3-36, a peptide secreted by the gut in response to a meal, and reduces appetite
- YY (social network), a major Chinese social network
- Yy (digraph), digraph used in various Latin alphabets
- Yy (musician), Canadian musician
- IATA code for all airlines in a booking, (Used to avoid duplicating requests to individual airlines).