John Louis O'Sullivan (November 15, 1813 - March 24, 1895) was a American newspaper journalist who is thought to have coined the term "manifest destiny" to encourage US expansionism. In subsequent years, it became the justification for the US annexations of the Mexican Cession, which expanded the US to the Pacific Ocean.

O'Sullivan was the founder and editor of the United States Magazine and Democratic Review (1837-1846). He was also the editor of the New York Morning News (1844-1846).

In 1854 O'Sullivan was appointed ambassador to Portugal, a post he held until 1858.

See further: Mexican-American War