In the metric system, a microgram or microgramme is a unit of mass equal to one millionth (1×10−6) of a gram.

In μg, the prefix symbol for micro- is the Greek letter μ (mu).

The United States–based Institute for Safe Medication Practices (ISMP) and the U.S. Food and Drug Administration (FDA) recommend that the symbol μg should not be used when communicating medical information due to the risk that the prefix μ (micro-) might be misread as the prefix m (milli-), resulting in a thousandfold overdose.

Gamma (symbol: γ) is a deprecated non-SI unit of mass equal to 1 μg.

[2] A fullwidth version of the "microgram" symbol is encoded by Unicode at code point U+338D ㎍ SQUARE MU G for use in CJK contexts.