Three fire engines and two fire rescue units and around 30 firefighters attended a person in the water near Tagg's Island in Hampton.
Station Manager Lee Conlon, who was at the scene said:
“Firefighters carried out a methodical search and despite the best efforts all emergency service crews in attendance the 14-year-old boy was not located.”
The Brigade was called at 1640. The rescue operation was concluded at 1840.
The Police have confirmed that the boy’s next of kin has been informed and are being supported by specialist officers.
Assistant Commissioner Charlie Pugsley said:
“Our thoughts are with the teenager’s family and friends. Please think twice before going into open water during this heatwave it can be incredibly dangerous as the water is a lot colder than you’d expect.
“Anything below 15°C is defined as cold water, and on average the UK and Ireland sea temperatures are just 12°C. Rivers such as the Thames are colder - even in the summer.”