A whistleblower (also written as whistle-blower or whistle blower) is a person, often an employee, who reveals information about activity within a private or public organization that is deemed illegal, immoral, illicit, unsafe or fraudulent. Whistleblowers can use a variety of internal or external channels to communicate information or allegations. Over 83% of whistleblowers report internally to a supervisor, human resources, compliance, or a neutral third party within the company, hoping that the company will address and correct the issues. A whistleblower can also bring allegations to light by communicating with external entities, such as the media, government, or law enforcement. Whistleblowing can occur in either the private sector or the public sector.

Retaliation is a real risk for whistleblowers, who often pay a heavy price for blowing the whistle. The most common form of retaliation is abrupt termination of employment. However, several other actions may also be considered retaliatory, including extreme increases in workloads, having hours cut drastically, preventing task completion, or bullying. Laws in many countries attempt to protect whistleblowers and to regulate the whistleblowing activities. These laws tend to adopt different approaches to public and private sector whistleblowing.

Whistleblowers do not always achieve their aims. For their claims to be credible and successful, they must have compelling evidence to support their claims that the government or regulating body can use or investigate to "prove" such claims and hold corrupt companies and/or government agencies to account.