The jury's conclusion comes following an inquest at the at the Old Bailey court in London. Britain's chief coroner Mark Lucraft QC was heard saying the armed officer who shot Masood three times with a Glock pistol "honestly believed it was necessary" in defense of himself and others.
CCTV footage of Masood, carrying two large knives and approaching two armed close protection officers coming out of the Parliament, was shown to the jury.
The police officer, who cried during his testimony, told the court he screamed at Masood to drop his weapon, but the assailant dismissed it.
"He was going to kill me," the officer said.
After he was shot, Masood received first aid treatment but died at the scene. Three gunshot wounds were found during an autopsy, the inquest found, with the first bullet, striking his upper torso and believed to have caused his death.
52-year-old Khalid Masood drove a vehicle into pedestrians on Westminster bridge on March 22, 2017. In the terror attack, dozens were injured and five people died, including an unarmed police officer — PC Keith Palmer stabbed by Masood.
Masood, who was born Adrian Elms, was known to the authorities and had come under the radar of MI5 several years ago as a "peripheral" figure in a terrorism investigation, Prime Minister Theresa May told MPs a few days after the attack.