Officers were called to reports of boy in the water just before 5.30pm at Stocking Lane.
The 15-year-old was pulled from the water but he was sadly pronounced dead at the scene.
His family are being supported by specialist officers.
It comes after the body of a man was recovered at Pugneys Country Park on Monday, following a two-day search of the lake.
A man in his 50s was also pulled from the River Ouse in York on Sunday after kayakers spotted his body near the Water End Bridge
And a man’s body was pulled from the lake at Crookes Valley Park in Sheffield on the same day.
An investigation has been launched by West Yorkshire Police’s Homicide and Major Enquiry Team following the Knottingley incident.
Detectives want to hear from anyone who was in the area at around the time of the incident, or who may have information about how the boy ended up in the water.
Detective Chief Inspector Tony Nicholson, said: “This has been a tragic incident in which a boy has sadly lost his life and I would like to advise people of the dangers posed by open water.
“Even on days as hot as we have had recently, canals can still be very cold and cold water can cause cramps in even the strongest swimmers.
“Currents created by boats and water movements around locks and weirs can also be dangerous. There may also be items lurking beneath the surface that could cause injury.”
Call 101 or use the options on the West Yorkshire Police website.
The log reference is 1462 of 20 July.