The body is identified to be a man and he was pronounced dead at 3.40pm.
The police said they received a call asking for assistance at around 3.10pm.
The case is classified as unnatural death and investigations are ongoing.
Serangoon Reservoir was officially opened only last Sunday, and a resident told Yahoo! Singapore that the area is popular with anglers.
Said Ismail Said, 57, "You can often see anglers risking their lives by going down to the bank to fish."
In a separate incident, a woman was arrested on Monday night in connection to the decomposed body found off Clementi Road last week.
The suspect is a 36-year-old woman and she is believed to have known the victim.
According to Channel NewsAsia, the victim has been identified to be Celine Ng Swee Peng, 37, who has been reported missing since 26 May.
The suspect will be charged with murder on Wednesday.
