St. Tammany Parish (French: Paroisse de Saint-Tammany; Spanish: Parroquia de St. Tammany) is a parish located in the U.S. state of Louisiana named after Tamanend, the legendary Lenape Chief of Chiefs and the "Patron Saint of America." At the 2020 census, the population was 264,570, making it the fourth-most populous parish in Louisiana. The parish seat is Covington. The parish was founded in 1810.

St. Tammany Parish comprises the Slidell–Mandeville–Covington metropolitan statistical area, which is also included in the New Orleans–Metairie–Slidell combined statistical area. St. Tammany Parish is one of the fastest-growing parishes in the state, along with Livingston and Ascension. The population has quadrupled since 1970, and is expected to double again by 2030, expecting to diversify the population of the parish. Though it was not heavily directly damaged by Hurricane Katrina, the community is growing in large part due to subsequent displacement of populations, with many coming from the New Orleans metropolitan area seeking to avoid hurricane and weather-related threats, caused in part due to climate change.

St. Tammany was originally inhabited by numerous Indian peoples, including the Colapissas, Bayou Goulas, Chickasaw, Biloxi, Choctaw and Pensacola nations (although Frederick S. Ellis, in his book St. Tammany Parish: L'autre Côté du Lac, claims that the regionally prominent Choctaw tribe did not arrive in the area until after it had begun to be settled by Europeans).

In 1699, Pierre Le Moyne d'Iberville, a French explorer, was the first European to visit the area of present-day St. Tammany Parish. While exploring lakes Pontchartrain and Maurepas, Iberville wrote in his journal, "The place where I am is one of the prettiest I have seen, fine level ground bare of canes. The land north of the lakes is a country of pine trees mixed with hard woods. The soil is sandy and many tracks of buffalo and deer can be seen."

After the 18th-century founding and development of New Orleans, French settlers began to enter the region. The primary industry was the production of pitch, tar, turpentine and resin from the forests.

After France was defeated in the French and Indian War, St. Tammany (along with the other future "Florida Parishes") became part of British West Florida. During this period, the area comprising today's St. Tammany attracted British loyalists who wanted to escape persecution in the Thirteen Colonies. After Great Britain was defeated in the American Revolutionary War, West Florida was governed by the Spanish. The West Florida period ended with the West Florida revolt, which precipitated annexation by the United States.

In 1810, President James Madison claimed West Florida as part of Louisiana and sent William C. C. Claiborne to claim the territory. Claiborne established the boundaries of the Florida Parishes. He created St. Tammany Parish and named it after the Delaware Indian Chief Tamanend (c.1628-1698), who made peace with William Penn and was generally renowned for his goodness. Among the nine Louisiana parishes (counties) named for "saints" (see "List of parishes in Louisiana"), St. Tammany is the only one whose eponym is not a saint of the Roman Catholic Church, the ecclesiastical parishes of which formed the basis for the state's civil parishes. In fact, Tamanend is not known to have been Christian, or specifically Roman Catholic. However, he became popularly revered as an "American patron saint" in the post-Revolutionary period (long after his death).

Under Spanish rule, the area east of the Tangipahoa River, which would become St. Tammany Parish (and later Washington Parish to the north), was known as the St. Ferdinand District. Due to the fact that Ferdinand was also the name of the disputed King of Spain at the time, it was deemed that the new parish should have a more "American" sounding name.