The actor detailed his experience with the unknown during a recent interview with Late Show host Stephen Colbert.
Harrelson, who recently performed in an alien abduction sketch on SNL50, recollected a childhood experience with a UFO that had remained firmly entrenched in his memory.
"Yeah. I mean, I'm open-minded," he told host Stephen Colbert on The Late Late Show.
"If you've been abducted, now would be a great time to tell me!" Colbert quipped. "But have you like seen a UFO? 'Cause I have, I've seen stuff I can't explain."
Harrelson went on to describe what happened to him as a teenager living in Ohio in 1974.
"Suddenly I noticed, like everybody's out in the street and so I went out, and we look up," he said.
"It was nighttime and there were these lights that were just kind of blinking and then it would just shoot all the way across the sky. And then there'd be one over there, shoot over here, and there were several."
"They just kept going across the sky."
The actor found it particularly strange that nobody seemed to want to talk about it afterward.
"Nobody talked about it," he said. "Literally, everybody just went inside and said nothing."
"It was so freaky, even for me, as young as I was."
On the topic of aliens, Harrelson remains open-minded.
"Why wouldn't it be true ?" he said.
You can check out the interview below (jump to 2:00 to hear his story).