P9 or P-9 may refer to:
- Planet Nine, a hypothetical new planet in the far outer solar system
- Boeing XP-9, a prototype fighter aircraft from the 1930s
- Heckler & Koch P9, a semi-automatic pistol
- Springfield Armory P9, a semi-automatic pistol
- Peruvian Airlines IATA airline designator
- P9 (band), a Brazilian boyband
- Power Nine, a trading cards game of Magic: The Gathering
- Psyclon Nine, an aggrotech band
- P-9 Project