The Single Bullet Theory (or Magic Bullet Theory, as it is commonly called by its critics) was introduced by the Warren Commission in its investigation of the assassination of President John F. Kennedy to explain what happened to the bullet which struck Kennedy in the back and exited through his throat. Given the lack of damage to the presidential limousine consistent with it having been struck by a high-velocity bullet and the fact that Texas Governor John Connally was wounded and was seated directly in front of the president, the Commission concluded they were likely struck by the same bullet.

The theory, generally credited to Warren Commission staffer Arlen Specter (later a United States Senator from Pennsylvania), posits that a single bullet, known as "Warren Commission Exhibit 399" (also known as "CE 399"), caused all the wounds to the governor and the non-fatal wounds to the president.

According to the Single Bullet Theory, a three-centimeter (1.2")-long copper-jacketed lead-core 6.5-millimeter rifle bullet fired from the sixth floor of the Texas School Book Depository passed through President Kennedy’s neck and Governor Connally’s chest and wrist and embedded itself in the Governor’s thigh. If so, this bullet traversed 15 layers of clothing, 7 layers of skin, and approximately 15 inches of tissue, struck a necktie knot, removed 4 inches of rib, and shattered a radius bone. The bullet was found on a gurney in the corridor at the Parkland Memorial Hospital, in Dallas, after the assassination. The Warren Commission found that this gurney was the one that had borne Governor Connally. This bullet became a key Commission exhibit, identified as CE 399. Its copper jacket was completely intact. While the bullet's nose appeared normal, the tail was compressed laterally on one side.