On an episode of Ellen scheduled to air Monday, the Prisoners star revealed the gross-out wound after the talk show host asked him about the incident.
"A mirror broke in the middle of the scene, I hit a mirror and it broke," Gyllenhaal explained. "And it cut my hand."
"Did you punch it?," DeGeneres continued.
'No, I looked at it really hard," he joked with a smile. "It broke and then my hand was hurt."
'Do you want to see it? It's gross," he asked.
And it certainly was.
Gyllenhaal flipped his hand over to reveal a deep stitched-up gash, while the audience gasped in the background.
'That was a mixture of excitement and horror!' Jake said in between laughs.
"I was in the middle of a scene and then I cut it and had to go to the hospital," he later went on.
The injury occurred while he was filming an "emotionally charged" scene for the upcoming Nightcrawler, which also stars Bill Paxton and Rene Russo.
Gyllenhaal, who is also a producer on the film, lost 20 pounds to play a crime scene journalist in the crime drama.
Nightcrawler is set to hit theaters next year.