What appeared to be the carcass of a dolphin washed up on a beach at East Coast Park yesterday afternoon.
The area was deserted when The Straits Times arrived at about 8pm.
It is unclear how the dolphin's carcass, which was slightly less than 2m long, had got there.
But dolphins are not uncommon in waters off Singapore's southern shore.
The Straits Times understands that both the National Parks Board and Lee Kong Chian Natural History Museum are aware of the dolphin's carcass, and will decide what to do about it today.
There have been previous cases of marine mammals being found in local waters.
In July last year, a sperm whale's 10m-long carcass was found floating off Jurong Island. The species had never previously been found in the waters around Singapore or peninsular Malaysia.
Last month, a dead green sea turtle was found at Pulau Tekukor, one of Singapore's southern islands.
Mr Stephen Beng, chairman of the marine conservation group of the Nature Society (Singapore), said more marine mammals and reptiles have been spotted in Singapore waters.
"These animals must surface to breathe and are exposed to many risks, such as boat strikes, ingesting plastic and being entangled in discarded fishing nets," he said.
"With the death of the turtle, and now this dolphin, it is important to find out the cause of death so we can monitor and reduce threats to our marine environment which are under our control."