Mary Loretta Landrieu (pronounced /ˈlændruː/, LAN-droo; born November 23, 1955) is the senior United States Senator from the State of Louisiana, and is the second woman elected to the U.S. Senate from Louisiana. Landrieu is the daughter of former New Orleans mayor and Secretary of the United States Department of Housing and Urban Development, Moon Landrieu and the sister of the current Mayor of New Orleans and former Lieutenant Governor of Louisiana Mitch Landrieu. By national standards, Landrieu is among the most conservative Democrats in the U.S. Senate, and is a member of the New Democrat Coalition.

Landrieu was born in Arlington, Virginia to Verna Satterlee and former New Orleans mayor Moon Landrieu, and raised in New Orleans. She was raised as a Roman Catholic and attended Ursuline Academy of New Orleans. She graduated from Louisiana State University in Baton Rouge in 1977 where she was a member of Delta Gamma sorority. Before entering politics, she worked as a real estate agent. She was a member of the Louisiana House of Representatives from 1980 to 1988, representing a New Orleans-based district. She then served as Louisiana state treasurer from 1988–1996, having succeeded the veteran officeholder, Mary Evelyn Parker. In the 1987 primary for treasurer, she defeated two legislative colleagues, Kevin P. Reilly, Sr., chief executive officer at the time of Lamar Advertising Company in Baton Rouge, and Claude "Buddy" Leach, a former U.S. representative and the current chairman of the Louisiana Democratic Party. Landrieu gave up the treasurer's office to run for governor of Louisiana in 1995, but finished third in the state's jungle primary (which at that time was considered the actual election in Louisiana). The eventual winner was Democrat-turned-Republican Murphy J. "Mike" Foster, Jr.

Landrieu and her husband, attorney Frank Snellings (born 1949), who grew up in Monroe, have two children, Connor and Mary Shannon. Her son Connor is currently an enrolled student at Tulane University in New Orleans.