Marianne Deborah Williamson (born July 8, 1952) is an American self-help author, speaker, and political activist. She began her professional career as a spiritual leader of the Church of Today, a Unity Church in Warren, Michigan. Williamson has written several self-help books, including A Return to Love: Reflections on the Principles of A Course in Miracles in 1992, which became a New York Times Best Seller. She rose to prominence through frequent appearances on Oprah Winfrey's show, and becoming known as her "spiritual advisor".

Williamson ran unsuccessfully as an independent for California's 33rd congressional district in the United States House of Representatives in 2014, finishing fourth with 13.2% of the vote. She ran for the Democratic nomination for president in 2020, eventually dropping out and endorsing Bernie Sanders. She ran for president again in 2024, challenging incumbent President Joe Biden. After having suspended and restarted her campaign, Williamson ended her longshot White House bid in July 2024. She unsuccessfully campaigned for the position of DNC chair in 2025.

Williamson has been actively involved with charity work, founding such organizations as Center for Living in 1987, Project Angel Food in 1989, and the Peace Alliance in 1998. Allegations of abusive and bullying behavior toward both colleagues and staff have followed her throughout her career.

Marianne Deborah Williamson was born in Houston, Texas, on July 8, 1952. She is the youngest of three children of Samuel "Sam" Williamson, a World War II veteran and immigration lawyer, and Sophie Ann Kaplan, a homemaker and community volunteer.

Williamson was raised in an upper-middle-class family that practiced Conservative Judaism. Her family attended Congregation Beth Yeshurun. She learned about world religions and social justice at home and became interested in public advocacy when she saw her rabbi speak against the Vietnam War.

In 1965, after Williamson came home from school in the seventh grade, she recounted to her parents that her teacher supported the Vietnam War. Her father reacted by taking the family to Vietnam to help explain to Marianne why he thought that the war was wrong. She has said that through travel she "had an experience, at a young age, that people are the same everywhere."

Williamson attended Houston ISD's Bellaire High School. After graduating, she spent two years studying theater and philosophy at Pomona College in Claremont, California, where she was a roommate of future film producer Lynda Obst. In 1973, Williamson dropped out of college and lived "a nomadic existence" during what she calls "her wasted decade".

Williamson moved to New Mexico, where she took classes at the University of New Mexico. The couple broke up a year later. Marianne then moved to Austin, Texas, where she took classes at the University of Texas. After leaving Texas, she went to New York City, intending to pursue a career as a cabaret singer; however, she has stated that she was distracted by "bad boys and good dope". Vanity Fair wrote that Williamson "spent her twenties in a growing state of existential despair." In New York, Williamson suffered from deep depression following the end of a relationship. She has said that this experience gave rise to a desire to spend the rest of her life helping people. Allegations of abusive behavior toward colleagues and staff have followed her throughout her career.