Officers were called to the River Calder in Brighthouse at 2.38pm on Tuesday.
After a search of the area, the body of the boy was found by West Yorkshire Police.
Police confirmed 30-50 teenagers had been swimming in the water.
Chief Superintendent Sarah Baker, district commander of Calderdale Police, said in a statement: “Whilst investigations into what took place are at an early stage, indications are that this is a tragic incident in which a teenage boy has lost his life.
“Specially trained officers are working to support the victim’s family at this devastating time, and our thoughts are with them.
“I would ask anyone who saw what happened and who may have not yet spoken with authorities, to contact Calderdale CID on 101 referencing police log 989 of June 1st.”
This is a breaking news story, more to follow…