Michael Rawlins, originally of Elmwood Park, was out for a hike in Pompton Lakes when he saw the bones floating in the river.
Hackensack Riverkeeper Captain Bill Sheehan was able to identify it by its teeth.
"You can clearly see the grinding teeth, which are like molars for dogs or humans," Sheehan told Daily Voice. That's what deer have because they're vegetarians and need to be able to grind food up."
Sheehan said it's not uncommon to find animal skeletons in bodies of water, noting the deer may have been shot by a hunter and escaped, or been hit by a car and gone back into the woods -- only to die in the river.
"A fact of life is that things do die," he said. "It's hard for wildlife in New Jersey."
Click here to follow Daily Voice Northern Highlands and receive free news updates.