Marianne Deborah Williamson (born July 8, 1952) is an American spiritual teacher, author and lecturer. She has published eleven books, including four New York Times number one bestsellers. She is the founder of Project Angel Food, a meals-on-wheels program that serves homebound people with AIDS in the Los Angeles area, and the co-founder of The Peace Alliance, a grassroots campaign supporting legislation to establish a United States Department of Peace. She serves on the Board of Directors of the RESULTS organization, which works to end poverty in the United States and around the world. Williamson is also behind Sister Giant, a series of seminars and teaching sessions that provides women with the information and tools needed to be political candidates. Through these seminars, she encourages women to run for office and align their politics with their spiritual values.

She has been a guest on television programs such as The Oprah Winfrey Show, Larry King Live, Good Morning America, Charlie Rose and Real Time with Bill Maher. In December 2006, a Newsweek magazine poll named her one of the fifty most influential baby boomers. According to Time magazine, "Yoga, the Cabala and Marianne Williamson have been taken up by those seeking a relationship with God that is not strictly tethered to Christianity." Williamson bases her teaching and writing on a set of books called A Course in Miracles, a self-study program of spiritual psychotherapy, based on universal spiritual themes.

Williamson has sold a combined total of more than 3,000,000 books, a canon which led New York Times reporter Mark Leibovich to label her a "self-help guru".

On October 20, 2013, she announced her candidacy for California's 33rd Congressional District in the race to fill Representative Henry Waxman's seat after his retirement. In the California primary election of June 2014, Williamson came in fourth with 12.9% of the vote.