A whistleblower (whistle-blower or whistle blower)  is a person who tells the public or someone in authority about alleged dishonest or illegal activities (misconduct)  occurring in a government department or private company or organization. The alleged misconduct may be classified in many ways; for example, a violation of a law, rule, regulation and/or a direct threat to public interest, such as fraud, health/safety violations, and corruption. Whistleblowers may make their allegations internally (for example, to other people within the accused organization) or externally (to regulators, law enforcement agencies, to the media or to groups concerned with the issues).

One of the first laws that protected whistleblowers was the 1863 United States False Claims Act (revised in 1986), which tried to combat fraud by suppliers of the United States government during the Civil War. The act encourages whistleblowers by promising them a percentage of the money recovered or damages won by the government and protects them from wrongful dismissal.

Whistleblowers frequently face reprisal, sometimes at the hands of the organization or group which they have accused, sometimes from related organizations, and sometimes under law.

Questions about the legitimacy of whistle blowing, the moral responsibility of whistle blowing, and the appraisal of the institutions of whistle blowing are part of the field of political ethics.