On Wednesday noon, Mr Chua Tiam Seng, 49, and his friend were setting up a cage to capture crabs on Pulau Ubin when something near the pond caught his eye.
He went closer, and discovered that it was a rotting, human corpse. He suspected that the body was several days old, and called the police.
When he returned, a metre-long monitor lizard was feasting on the corpse.
A police spokesman said they have classified the case as unnatural death and investigations are ongoing.