The Banzhaf power index, named after John Banzhaf, is a power index defined by the probability of changing an outcome of a vote where voting rights are not necessarily equally divided among the voters or shareholders.

John Francis Banzhaf III is an American public interest lawyer, legal activist and law professor at George Washington University Law School. He is the founder of an antismoking advocacy group, Action on Smoking and Health. He is noted for his advocacy and use of lawsuits as a method to promote what he believes is the public interest.