A stowaway (also stoweaway) is a person who travels illegally, by airplane, ship or trains.

Stowaways face dangerous situations. Since they are not legally on board, they must sometimes spend days without water or food when travelling by ship, risking death. An equal risk of death is taken when trying to board an airplane. Usually, a stowaway tries to jump into an airplane by hanging on to the airliner's landing gears as the plane takes off, and the impact that the velocity of the airplane added to the power of the wind cause could easily make a stowaway fall to death. Such was the case of a person trying to fly from Kingsford Smith International Airport in Sydney, Australia to Tokyo, Japan, on a Japan Airlines Boeing 747, when he fell to his death during take-off. The photo of this incident became famous. Because people flying on airplanes as stowaways must stay within the landing gear area, they face other risks, such as falling when the plane is landing, or dying from the heat produced by the wings and engines of the airplane.

Stowaways also risk facing jail time, as it is illegal on most places to embark on airplanes, boats or trains as stowaways. Airports, Sea ports and Train stations are either operated by the government or by private groups, so trespassing these structures to get on board is usually punishable by law.

There are several different reasons for which a person might try to become a stowaway, among them economic, immigration and political reasons. Some also become stowaways as a dare or a way to get a thrill.

Many (if not most) stowaways get on board without knowing where they will end up arriving to.

On May 9, 2004, two stowaways were severely injured when their flight, American Eagle Flight 1450 from Mayagüez to Luis Muñoz Marín International Airport in San Juan, crashed during landing. They were able, however, to recuperate.

Since the September 11, 2001 terrorist attacks, it has become more difficult to be a stowaway in American airline flights. Airport security has increased, and among the new security measures is watching over the fences from which stowaways usually gain entrance to an airport's runway.

There are three films named Stowaway .