John Slidell (1793 - 1871) was a senator and diplomat to Mexico during the Mexican-American War. He was sent to Mexico by President Polk in 1846 to negotiate an agreement whereby the Rio Grande River would be the southern border of Texas. He was rejected and the United States declared war on Mexico on 13 May 1846.

During the Democratic Convention in Charleston, S.C. in April 1860, he plotted with "fire-eaters" such as Mr. William Yancey to defeat the only candidate who could have won the general election, namely, Mr. Stephen A. Douglas.

John Slidell was one of the two CSA diplomats involved in the Trent Affair in November, 1861. After having been appointed the Confederate States of America's commissioner to France in September, 1861, he ran the blockade from Charleston, S.C. With James M. Mason, he set sail from Havana on the British mail steamer Trent after which the Trent Affair is named.

After the Trent Affair was resolved, the two Confederate States of America diplomats set sail for Europe on January 1, 1862; he died in London.

The city of Slidell, Louisiana is named after him. He was the brother of Alexander Slidell Mackenzie, a naval officer who commanded the USS Somers on which a unique event occurred in 1842 off the coast of Africa during the Blockade of Africa. In that incident, three crewmen were hanged after being convicted of mutiny at sea.