Martha Helen Stewart (née Kostyra, Polish: [kɔˈstɨra]; born August 3, 1941) is an American retail businesswoman, writer, and television personality. As the founder of Martha Stewart Living Omnimedia, focusing on home and hospitality, she gained success through a variety of business ventures, encompassing publishing, broadcasting, merchandising and e-commerce. She has written numerous bestselling books, was the publisher of Martha Stewart Living magazine and hosted two syndicated television programs: Martha Stewart Living, which ran from 1993 to 2004, and The Martha Stewart Show, which ran from 2005 to 2012.

In 2004, Stewart was convicted of felony charges related to the ImClone stock trading case; she served five months in federal prison for fraud and was released in March 2005. There was speculation that the incident would effectively end her media empire, but in 2005 Stewart began a comeback campaign, and her company returned to profitability in 2006. Stewart rejoined the board of directors of Martha Stewart Living Omnimedia in 2011 and became chairwoman of her namesake company again in 2012. The company was acquired by Sequential Brands in 2015. Sequential Brands Group agreed in April 2019 to sell Martha Stewart Living Omnimedia, including the Emeril brand, to Marquee Brands for $175 million with benchmarked additional payments.

In 2023, at the age of 81, Stewart became the oldest woman to appear on the cover of Sports Illustrated Swimsuit Issue magazine. In 2024, she was the subject of a Netflix documentary titled Martha directed by R.J. Cutler.