Sussex Police said a search is underway after the boy was swept away by the sea whilst swimming with friends at West Beach, Newhaven, on Sunday afternoon.
Police, the coastguard and an in-shore lifeboat are searching for the teenager, who has not been named.
A Sussex Police spokesman said: “This is a very distressing incident and everything possible is being done to try to find the boy.”
Updates on the search will be issued as soon as they are available, the force said.
‘Stay clear’
Winds of more than 80 miles per hour are expected as the biggest storm in 25 years, Storm St Jude, arrives in the UK overnight.
Sussex Police Chief Inspector Katy Wolford said people have to “stay clear” of the water’s edge.
“Both we and the coastguard have received reports during the afternoon of people being dangerously near water edge in places as far apart as Seaford and Brighton,” she said.
“Please, at least until the imminent severe weather abates, stay clear of the seashore for your own safety, and in order to help the emergency services to be available to deal with other incidents.”
Amber warning
The impact of the storm has been felt elsewhere on Sunday. A family of four escaped unhurt when a tree crashed through the roof of their home in Cambourne, Cornwall, early on Sunday morning.
An 89ft wind turbine has also blown over in a field near Teignmouth, Devon.
The Met Office has issued an amber warning for southern parts of the country, meaning people should be prepared. Heavy rainfall is expected to accompany the high winds rain the storm hits overnight.