A stowaway is a person who secretly boards a vehicle, such as a ship, an aircraft, a train, cargo truck or bus, without the consent of the owner or the responsible person. The purpose of stowing away is sometimes to get from one place to another without paying for transportation, while in other cases, it is to enter another country without first obtaining a travel visa or other permission. Stowaways differ from people smuggling in that the stowaway needs to avoid detection by the crew or others responsible for the safe and secure operation of the transportation service. The Convention on Facilitation of International Maritime Traffic, 1965, as amended, defines a stowaway as "A person who is secreted on a ship, or in cargo which is subsequently loaded on the ship, without the consent of the shipowner or the Master or any other responsible person and who is detected on board the ship after it has departed from a port, or in the cargo while unloading it in the port of arrival, and is reported as a stowaway by the master to the appropriate authority".

Stowaways sometimes hide themselves in vehicles such as cargo trucks to get between cities, although this is also done by migrants who have paid the driver. In some places, drivers are legally responsible for making sure that stowaways do not board their vehicles, and can be fined or jailed if they do not detect and remove a stowaway when crossing national borders.

The majority of stowaways come from impoverished or war-torn countries or areas of conflict, and the problem is global, but many stowaways are from the poorer Asian and African countries and are looking to build a new life elsewhere.