John Francis Banzhaf III (pronounced /ˈbænz.hɑːf/) (born July 2, 1940) is a legal activist and a law professor at George Washington University Law School. He is the founder of the US smoking pressure group Action on Smoking and Health. He is noted for his advocacy of, and use of, lawsuits as a method to promote the public interest.

Banzhaf was born July 2, 1940 in New York City. He graduated from Manhattan's Stuyvesant High School at the age of 15, worked for a year with an electrical engineering firm, then entered MIT where he received his BSEE degree in 1962. He had already published several papers in electrical engineering and been awarded several patents. A few months after graduation he entered Columbia University Law School, intending to specialize in patent law. He graduated with a JD degree in 1965.

After doing some research and law clerk work, he became an associate with the patent-law firm Watson, Leavenworth, Kelton & Taggart. In 1968 he was appointed associate professor at the National Law Center (since renamed George Washington University Law School) of George Washington University, and was promoted to full professor with tenure in 1971.