155 (one hundred [and] fifty-five) is the natural number following 154 and preceding 156.

In mathematics

155 is:

• a composite number
• a semiprime.
• a deficient number, since 1+5+31=37<155.
• odious, since its binary expansion ${\displaystyle 10011011_{2))$ has a total of 5 ones in it.

There are 155 primitive permutation groups of degree 81. OEIS: A000019

If one adds up all the primes from the least through the greatest prime factors of 155, that is, 5 and 31, the result is 155. (sequence A055233 in the OEIS) Only three other "small" semiprimes (10, 39, and 371) share this attribute.

In the military

• USS Capable (AM-155) was a United States Navy Admirable-class minesweeper during World War II
• USS Cole (DD-155) was a United States Navy Wickes-class destroyer during World War II
• USS General A. W. Brewster (AP-155) was a United States Navy General G. O. Squier-class transport during World War II
• USS Hopping (DE-155) was a United States Navy Buckley-class destroyer escort during World War II
• USS Lycoming (APA-155) was a United States Navy Haskell-class attack transport during World War II
• USS Mustang (IX-155) was a United States Navy four-master wooden schooner during World War II
• 155mm artillery, a common caliber

In sports

• The maximum possible score in a single break in snooker, with a free ball at the start of the break (147 is the highest possible without a free ball)

In transportation

• Alfa Romeo 155, a compact executive car produced from 1992 to 1998
• Seattle Bus Route 155
• London Bus Route 155
• The British Rail Class 155 is a diesel multiple unit British train
• Blériot 155 was a French airliner of the 1920s
• 155th Street, Manhattan, New York
• 155th Street New York City Subway stations:
  • 155th Street (IND Concourse Line)
  • 155th Street (IND Eighth Avenue Line)
  • 155th Street (IRT Ninth Avenue Line) (former)

In other fields

155 is also:

• The year AD 155 or 155 BC
• 155 is the number for the International Operator in the United Kingdom
• 155 AH is a year in the Islamic calendar that corresponds to 771–772 CE
• 155 Scylla is a main belt asteroid
• Europium-155 is a radioisotope or Europium and fission product with a half-life of 4.76 years
• 155, a 2007 song by the band +44
• The dialing code for Obihiro, Japan
• The atomic number of an element temporarily called unquintquinttium
• 155 North Wacker is a 46-story skyscraper in Chicago
• Wolseley No. 155, Saskatchewan, is a rural municipality in Canada