The augmented reality game, which was released last week, gets people to catch virtual monsters using the person's location on their phone.
Nineteen-year-old Shayla Wiggins, from Wyoming, was told to find a Pokemon in a natural water source but instead found a man's corpse.
"I was walking towards the bridge along the shore when I saw something in the water," she told County 10 news. "I had to take a second look and I realised it was a body."
Shayla, who's living in a trailer park for the summer with her mum, said she'd jumped over a fence to get to the Pokemon shown on her map.
"It was pretty shocking," she said. "I didn't really know what to do at first. But I called 911 right away and they came really quickly."
The body was found in the Big Wind River under the Wyoming Highway 789 Bridge. The local police department says there doesn't appear to be any foul play involved.
"The death appears to be accidental in nature and possibly that of a drowning," said the Fremont County Sheriff's Office in a statement. "There is no evidence at this time that would indicate foul play. Evidence located at the scene has led investigators to believe the man went into the water at the location he was found."
Pokemon Go has led to a spate of injuries in America. The game became the top grossing app in the iPhone store last week after its release in the US, Australia and New Zealand.
Players have already reported wiping out in a variety of ways as they wander the real world with eyes glued to their smartphone screens in search of digital monsters.
Mike Schultz, a 21-year-old communications graduate in Long Island, New York, fell over while looking for Pokemon on his skateboard. He cut his hand on the pavement after hitting a crack.
"I just wanted to be able to stop quickly if there were any Pokemons nearby to catch," he says. "I don't think the company is really at fault."