Finding a previously unidentified species in the ponds and marshes of Staten Island, mainland New York and New Jersey is a big surprise, they said.
"For a new species to go unrecognized for all this time in this area is amazing," UCLA Professor Brad Shaffer, one of the authors on the paper announcing the discovery, said.
The as-yet unnamed species has for years been mistaken by biologists for a more widespread species of leopard frog, he said.
"Many amphibians are secretive and can be very hard to find, but these frogs are pretty obvious, out-there animals," Shaffer said. "This shows that even in the largest city in the U.S. there are still new and important species waiting to be discovered that could be lost without conservation."
Researchers said they began investigating some "unusual frogs" whose weird-sounding calls were different from other leopard frogs.
"When I first heard these frogs calling, it was so different, I knew something was very off," researcher Jeremy Feinberg at Rutgers University said. "It's what we call a cryptic species: one species hidden within another because we can't tell them apart on sight.
It took DNA tests to confirm the "odd" croakers were in fact a new species, he said in a Rutgers release Wednesday.
Until the scientists settle on a new name, the new frog will be known as "Rana sp. nov.," meaning "new frog species."