The grisly discovery was made shortly after 9 a.m. when a local resident spotted the corpse floating in the murky waters near the Bronx River Park in Williamsbridge.
Police have yet to indentify the victim, whom they described as black and in her 20s or 30s. The medical examiner will conduct an autopsy to determine how she died.
One local resident who was among the first people to see the body said it was spotted across the river from a paved path that leads down to a line of small trees at the water's edge. The resident said there was blood on the path.
Dozens of people turned out to watch as police and officials from the city medical examiner's office fished the body out of the river about 1 p.m. and investigated the crime scene.
Detectives appealed to the crowd for any information that may help them as they investigate.
The part of the park that is close to where the body was found is parallel to Bronx Blvd., near E. 226th St.
There are two bocce courts in that part of the park, and neighbors said the area is a hangout where young people smoke pot and party.
Some neighbors expressed concern about a pervert who has recently been exposing himself to women in the area, particularly pregnant women.
One woman, who declined to give her name, said the man actually touched her stomach and asked if she was pregnant.
A man who lives in the area described the wooded, riverside park as "relatively safe," but said he still warns female relatives to avoid the park after dark.
"If you're out here late at night, you should know better," he said, declining to give his name.
Police ask anyone with information to call the Crime Stoppers tipline at 800-577-TIPS. All calls are confidential.