Boo may refer to:
- Fictional characters
- Boo (character), a ghost character in the Mario series
- Boo, a hamster in Megatokyo
- Boo, a baby human girl from Monsters, Inc.
- Boo, a hamster belonging to the Minsc in Baldur's Gate
- Majin Buu (魔人ブウ, Majin Bū?, also translated as "Majin Boo"), a magical life form in Dragon Ball
- Boo Radley, a character in To Kill a Mockingbird
- Boo! (comic strip), a character in the British comic The Dandy created by Andy Fanton
- Film and television
- Boo! (film), a 1932 comedy film
- Boo! (TV series), a 2003 BBC children's television series
- "Boo" (CSI: NY episode)
- "Boo! (a.k.a. I'm With Her)", a 2004 episode of Frasier
- "Boo" (Dark Angel), an episode of the television series Dark Angel
- Geography
- Boo (Aller), Spain
- Boo de Piélagos, Piélagos, Cantabria, Spain
- Boo de Guarnizo, El Astillero, Cantabria, Spain
- Boo, Sweden, in Stockholm County
- Boo, Guinea, in Nzérékoré Prefecture
- Music
- Boo! (album), by Was (Not Was)
- Boo! (band), a 1997–2004 and 2010–present South African band
- People with surname Boo
- Ben Boo (born 1925), American politician
- Katherine Boo (born 1964), American journalist and McArthur Fellow
- People with given name Boo
- Boo Cook (born 1972), British comics artist
- Boo Hewerdine (born 1961), British singer-songwriter
- Boo McLee (born 1983), American football player
- Boo Weekley (born 1973), American golfer
- Boo (dog)
- Other uses
- Boo (programming language)
- Boo.com, a clothing company
- Boötes constellation; standard abbreviation
- Better Off Out, a political campaign
- Boô, a Saxon cattle shed
- "Boo", a term of endearment
- BOO is the IATA airport code for Bodø Airport in Norway
- BOO, and acronym for Build Own Operate, a form of infrastructure project operating concession
- "Boo!", an exclamation depicted as being uttered by Ghosts to scare or surprise the living.
- "Boo" pet name: term of endearment for Poonam Patel (born 1983)