Police paid tribute to Temple Bowley, who discovered Dallas police officer J.D. Tippit lying dead in the street Nov. 22, 1963, less than an hour after the president was shot.
Investigators later said Tippit had stopped Oswald, accused in the JFK assassination, and was shot.
"You don't run upon a dead man every day," Bowley told The Dallas Morning News.
Bowley was en route to pick up his wife from work when he found Tippit's body in the street. After making the discovery, he climbed into the officer's car and used the police radio to report the shooting.
Officers arrived at the scene of the Tippit shooting, and shortly afterward they arrested Oswald at the nearby Texas Theatre.
Oswald initially was held for Tippit's death, not JFK's.
Police Chief David Brown was to present Bowley with a Citizen's Certificate of Merit, the newspaper said.
"I don't deserve the recognition," said Bowley, now an 82-year-old retiree. "It was just the thing to do. The radio was there and it was connected to the Police Department, and that's who I needed to talk to."
Bowley has another connection to the events surrounding the assassination, the Morning News said. He spent several years in the 1950s working as a doorman for Dallas nightclub operator Jack Ruby, who shot Oswald to death on live television two days after the Kennedy assassination.