Jack Tar (also Jacktar, Jack-tar or Tar) is a common English term that was originally used to refer to seamen of the Merchant Navy or the Royal Navy, particularly during the British Empire. By World War I the term was used as a nickname for those in the US Navy. Members of the public and seafarers alike made use of the name in identifying those who went to sea. It was not used pejoratively, and sailors were happy to use the term to label themselves.

There is some dispute among historians about the origin of "Jack", but it was a frequently used generic name for the common man. There are several plausible etymologies for the reference to "tar":

The Colledge will the whole world measure,

Which most impossible conclude,

And Navigation make a pleasure

By fynding out the Longitude.

Every Tarpaulin shall then with ease

Sayle any ship to the Antipodes.