In financial regulation, a Suspicious Activity Report (SAR) or Suspicious Transaction Report (STR) is a report made by a financial institution about suspicious or potentially suspicious activity. The criteria to decide when a report must be made varies from country to country, but generally is any financial transaction that does not make sense to the financial institution; is unusual for that particular client; or appears to be done only for the purpose of hiding or obfuscating another, separate transaction. The report is filed with that country's financial crime enforcement agency, which is typically a specialist agency designed to collect and analyse transactions and then report these to relevant law enforcement. Front line staff in the financial institution have the responsibility to identify transactions that may be suspicious and these are reported to a designated person that is responsible for reporting the suspicious transaction. This means that the front line staff can ask questions and, in some cases, even decline suspicious transactions. The financial institution is not allowed to inform the client or parties involved in the transaction that a SAR has been lodged, otherwise known as tipping off under the Financial Action Task Force's Recommendations.

The Financial Action Task Force's Recommendations are widely recognized as the international standard in anti-money laundering and countering financing terrorism with endorsements from 180 nations. FATF Recommendations set forth essential measures to combat money laundering and to protect domestic and international monetary systems including the application of preventive measures for the financial sector and other designated sectors; and establishment of powers and responsibilities for the relevant competent authorities (e.g., investigative, law enforcement and supervisory authorities), including guidelines regarding suspicious activity reports. Most countries have laws that require financial institutions to report suspicious transactions and will have a designated agency to receive them. The agency to which a report is required to be filed for a given country is typically part of the law enforcement or financial regulatory department of that country. For example, in the United States, suspicious transaction reports must be reported to the Financial Crimes Enforcement Network (FinCEN), an agency of the United States Department of the Treasury. In Australia the SAR must be reported to Australian Transaction Reports and Analysis Centre (AUSTRAC), an Australian government agency.