Officers were called to Fairlop Waters, in Redbridge, just after 12.30am on Monday, July 4, after reports a teenage boy had gone for a swim and failed to return to the shore.
Searches were carried out by the National Police Air Service, London Fire Brigade and the Marine Policing Unit.
Specialist divers also searched the area for the teenager, before his body was recovered from the water on Monday afternoon.
A Met Police spokesperson said: “The teenager’s family has been informed and are being supported by specialist officers.
“The death is currently being treated as unexpected.
“Fairlop Waters will remain closed to the public whilst our enquiries continue.”
The teenager’s identity has not yet been released.
A Redbridge council spokesperson said on Monday: “Following an incident on the lake at Fairlop Waters in the early hours of this morning, the park is closed to visitors at the request of the emergency services.
“At this moment we do not know when the park will be re-opened. Please check our website for further updates.”