The news service said Narciso Contreras recently told its editors that he manipulated a digital picture of a Syrian rebel fighter taken in September, using software to remove a colleague's video camera from the lower left corner of the frame.
That led AP to review nearly 500 photos Contreras has filed since he began working for the news service in 2012. No other instances of alteration were uncovered, said Santiago Lyon, the news service's vice-president and director of photography.
Contreras was one of a team of photographers working for the AP who shared in a Pulitzer last year for images of the Syrian war. None of the images in that package were found to be compromised, according to the AP.
The alteration breached AP's requirements for truth and accuracy even though it involved a corner of the image with little news importance, Lyon said.
"AP's reputation is paramount and we react decisively and vigorously when it is tarnished by actions in violation of our ethics code," said Lyon.
Contreras said he thought the video camera in the frame might distract viewers. But he added: "I took the wrong decision when I removed the camera … I feel ashamed about that," he said. "You can go through my archives and you can find that this is a single case that happened probably at one very stressed moment."