D. B. Cooper is a media epithet for an unidentified man who hijacked Northwest Orient Airlines Flight 305, a Boeing 727 aircraft operated by Northwest Orient Airlines, in United States airspace on November 24, 1971. During the flight from Portland, Oregon to Seattle, Washington, the hijacker told a flight attendant he was armed with a bomb, demanded $200,000 in ransom, (equivalent to $1,338,000 in 2021) and requested four parachutes upon landing in Seattle. After releasing the passengers in Seattle, the hijacker instructed the flight crew to refuel the aircraft and begin a second flight to Mexico City, with a refueling stop in Reno, Nevada. Approximately thirty minutes after taking off from Seattle, the hijacker opened the aircraft's aft door, deployed the staircase, and parachuted into the night over southwestern Washington. The hijacker was never identified, apprehended, or found.

In 1980, a small portion of the ransom money was found along the banks of the Columbia River. The discovery of the money renewed public interest in the mystery but yielded no additional information about the hijacker's identity or fate, and the remaining money was never recovered. The hijacker identified himself as Dan Cooper but, because of a reporter's mistake, became known as "D. B. Cooper".

For 45 years after the hijacking, the Federal Bureau of Investigation (FBI) maintained an active investigation and built an extensive case file, but ultimately reached no definitive conclusions and the crime remains the only unsolved case of air piracy in the history of commercial aviation. The FBI speculates Cooper did not survive his jump, for several reasons: the inclement weather on the night of the hijacking; Cooper's lack of proper skydiving equipment; his drop zone was a heavily wooded area; Cooper's apparent lack of detailed knowledge of his landing area; and the disappearance of the remaining ransom money, suggesting it was never spent. In July of 2016, the FBI officially suspended active investigation of the NORJAK (Northwest Hijacking) case, although reporters, enthusiasts, professional investigators, and amateur sleuths continue to pursue numerous theories for Cooper's identity, success, and fate.

Cooper's hijacking—and several imitators in the following year—led to immediate and major changes for commercial aviation and stricter airport security measures. Metal detectors were installed, baggage inspection became mandatory, and passengers who paid cash for tickets on the day of departure were selected for additional scrutiny. Aircraft were fitted with eponymous "Cooper Vanes," specifically designed to prevent the aft staircase from being lowered in-flight. By 1973, aircraft hijacking incidents had decreased as the new security measures successfully dissuaded would-be hijackers whose only motive was money.