Curious onlookers approached a cordon to take photographs of the humpback, whose 14-meter (45 feet) long carcass gave off a putrid stench as it lay exposed to the sun.
Some touched its jaw bones, which had come loose and lay on the damp sand.
Rafael Carvalho, a marine mammal biologist at the State University of Rio de Janeiro, told AFP: "It is not possible to know what happened, it is in an advanced state of decomposition and that makes it difficult to know the cause of death."
Carvalho is part of a team that is dedicated to removing beached whales, a relatively common phenomenon in the area because Rio lies along their migration path.
The body will be removed in a truck and then buried for sanitary reasons, he added.
"I've never seen one so close, I would have liked to have had the chance to save it," mused Mauro Azevedo, a 62-year-old Rio resident.
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