A tax protester is someone who refuses to pay a tax claiming that the tax laws are unconstitutional or otherwise invalid. Tax protesters are different from tax resisters, who refuse to pay taxes as a protest against a government or its policies, or a moral opposition to taxation in general, not out of a belief that the tax law itself is invalid. The United States has a large and organized culture of people who espouse such theories. Tax protesters also exist in other countries.

Legal commentator Daniel B Evans has defined tax protesters as people who "refuse to pay taxes or file tax returns out of a mistaken belief that the federal income tax is unconstitutional, invalid, voluntary, or otherwise does not apply to them under one of a number of bizarre arguments" (divided into several classes: constitutional, conspiracy, administrative, statutory, and arguments based on 16th Amendment and the "861" section of the tax code; see the Tax protester arguments article for an overview). Law Professor Allen D Madison has described tax protesters as "those who refuse to pay income tax on the basis of some nonsensical legal argument that he or she does not owe tax."

An illegal tax-protest scheme has been defined as "any scheme, without basis in law or fact, designed to express dissatisfaction with the tax laws by interfering with their administration or attempting to illegally avoid or reduce tax liabilities." The United States Tax Court has stated that "tax protester" is a designation "often given to persons who make frivolous antitax arguments".

Tax protesters raise a number of different kinds of arguments. In the United States, these typically include constitutional arguments, such as claims that the Sixteenth Amendment to the Constitution was not properly ratified or that it is unconstitutional generally, or that being forced to file an income tax return violates the Fifth Amendment privilege against self-incrimination. Others are statutory arguments suggesting that the income tax is constitutional but the statutes enacting the income tax are ineffective, or that Federal Reserve Notes or other relevant currencies do not constitute cash or income. Yet another collection of arguments centers on general conspiracies involving numerous government agencies.

Some tax protesters refuse to file a tax return or file returns with no income or tax data supplied.