55 (fifty-five) is the natural number following 54 and preceding 56.

55 is the 10th Fibonacci number^[1] and the 10th triangular number,^[2] where 10 is the sum of 55's digits. It's the 5th heptagonal number,^[3] the 5th square pyramidal number,^[4] and the 4th centered nonagonal number.^[5]

55 is also the 15th squarefree semiprime,^[6] and the 19th semiprime,^[7] as well as the 32nd nontotient number (including odd numbers > 1),^[8] the 36th arithmetic number,^[9] the 38th composite number^[10] (where the abundance of 55 = - 38),^[11] the 43rd deficient number,^[12] the 45th trapezoidal number ,^[13] and the 49th polite number.^[14]

• The atomic number of caesium.

• Messier object M55, a magnitude 7.0 globular cluster in the constellation Sagittarius
• The New General Catalogue object NGC 55, a magnitude 7.9 barred spiral galaxy in the constellation Sculptor

• The name of a song by Kasabian. The song was released as a B side to Club Foot and was recorded live when the band performed at London's Brixton Academy.
• "55", a song by Mac Miller
• "I Can't Drive 55", a song by Sammy Hagar
• "Ol' '55", a song by Tom Waits
• Ol' 55 (band), an Australian rock band.
• Primer 55 an American band
• Station 55, an album released in 2005 by Cristian Vogel
• 55 Cadillac, an album by Andrew W.K.

• In the United States, the National Maximum Speed Law prohibited speed limits higher than 55 miles per hour (90 km/h) from 1974 to 1987.

• 55 Days at Peking a film starring Charlton Heston and David Niven

• Gazeta 55, an Albanian newspaper
• Agitation and Propaganda against the State, also known as Constitution law 55, a law during Communist Albania.
• The code for international direct dial phone calls to Brazil
• A 55-gallon drum for containing oil, etc.
• The Élysée, the official residency of the French Republic president, which address is 55 rue du Faubourg-Saint-Honoré in Paris.

• 55th Regiment of Foot (disambiguation)
• Channel 55 (disambiguation)
• Type 55 (disambiguation)
• Class 55 (disambiguation)
• List of highways numbered 55