Mu or MU may refer to:

• mu (lowercase μ, uppercase Μ), a letter in the Greek alphabet
• Mu, a hypothetical lost continent in the Pacific Ocean
• mu (無), a Japanese word important in Zen koan practice
• mu (亩), Chinese unit of measurement equal to 666.7 square metres
• the IATA code for China Eastern Airlines
• Map unit (m.u.), used in genetics
• Mauritius (MU) ISO 3166-1 alpha-2 country code
• University of Missouri - Columbia (MU), "Missouri University"
• Monitor unit (MU), a unit used in radiation therapy
• multiple unit (MU), such as a rail diesel car
• University of Mumbai (MU) formerly called the University of Bombay.
• Mu-metal, an alloy used for screening magnetic fields
• a Japanese rocket, Mu (rocket)
• the Isuzu MU, a 2-Door Sports 4WD released in Japan.
• the Mirror Universe (MU), the dark alternate universe of Star Trek.

The lower-case letter μ is used as a symbol for:

• The SI prefix micro.
• The micron, old name for micrometre (10⁻⁶ m).
• In physics, the friction coefficient, mobility, and mass per unit length . Mu nought (μ₀) denotes the permeability of free space, which is exactly equal to 4π×10^-7 NA^-2.
• In electronics, the mutual gain of a vacuum tube.
• The arithmetic mean of a statistical population.
• mu is sometimes used to mean "the question contains an invalid assumption"